Re: [RFC PATCH v1 00/57] Boot-time page size selection for arm64

[Ryan Roberts <ryan.roberts@arm.com>]

On 21/10/2024 12:32, Eric Curtin wrote:
> On Mon, 21 Oct 2024 at 12:09, Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 19/10/2024 16:47, Neal Gompa wrote:
>>> On Monday, October 14, 2024 6:55:11 AM EDT Ryan Roberts wrote:
>>>> Hi All,
>>>>
>>>> Patch bomb incoming... This covers many subsystems, so I've included a core
>>>> set of people on the full series and additionally included maintainers on
>>>> relevant patches. I haven't included those maintainers on this cover letter
>>>> since the numbers were far too big for it to work. But I've included a link
>>>> to this cover letter on each patch, so they can hopefully find their way
>>>> here. For follow up submissions I'll break it up by subsystem, but for now
>>>> thought it was important to show the full picture.
>>>>
>>>> This RFC series implements support for boot-time page size selection within
>>>> the arm64 kernel. arm64 supports 3 base page sizes (4K, 16K, 64K), but to
>>>> date, page size has been selected at compile-time, meaning the size is
>>>> baked into a given kernel image. As use of larger-than-4K page sizes become
>>>> more prevalent this starts to present a problem for distributions.
>>>> Boot-time page size selection enables the creation of a single kernel
>>>> image, which can be told which page size to use on the kernel command line.
>>>>
>>>> Why is having an image-per-page size problematic?
>>>> =================================================
>>>>
>>>> Many traditional distros are now supporting both 4K and 64K. And this means
>>>> managing 2 kernel packages, along with drivers for each. For some, it means
>>>> multiple installer flavours and multiple ISOs. All of this adds up to a
>>>> less-than-ideal level of complexity. Additionally, Android now supports 4K
>>>> and 16K kernels. I'm told having to explicitly manage their KABI for each
>>>> kernel is painful, and the extra flash space required for both kernel
>>>> images and the duplicated modules has been problematic. Boot-time page size
>>>> selection solves all of this.
>>>>
>>>> Additionally, in starting to think about the longer term deployment story
>>>> for D128 page tables, which Arm architecture now supports, a lot of the
>>>> same problems need to be solved, so this work sets us up nicely for that.
>>>>
>>>> So what's the down side?
>>>> ========================
>>>>
>>>> Well nothing's free; Various static allocations in the kernel image must be
>>>> sized for the worst case (largest supported page size), so image size is in
>>>> line with size of 64K compile-time image. So if you're interested in 4K or
>>>> 16K, there is a slight increase to the image size. But I expect that
>>>> problem goes away if you're compressing the image - its just some extra
>>>> zeros. At boot-time, I expect we could free the unused static storage once
>>>> we know the page size - although that would be a follow up enhancement.
>>>>
>>>> And then there is performance. Since PAGE_SIZE and friends are no longer
>>>> compile-time constants, we must look up their values and do arithmetic at
>>>> runtime instead of compile-time. My early perf testing suggests this is
>>>> inperceptible for real-world workloads, and only has small impact on
>>>> microbenchmarks - more on this below.
>>>>
>>>> Approach
>>>> ========
>>>>
>>>> The basic idea is to rid the source of any assumptions that PAGE_SIZE and
>>>> friends are compile-time constant, but in a way that allows the compiler to
>>>> perform the same optimizations as was previously being done if they do turn
>>>> out to be compile-time constant. Where constants are required, we use
>>>> limits; PAGE_SIZE_MIN and PAGE_SIZE_MAX. See commit log in patch 1 for full
>>>> description of all the classes of problems to solve.
>>>>
>>>> By default PAGE_SIZE_MIN=PAGE_SIZE_MAX=PAGE_SIZE. But an arch may opt-in to
>>>> boot-time page size selection by defining PAGE_SIZE_MIN & PAGE_SIZE_MAX.
>>>> arm64 does this if the user selects the CONFIG_ARM64_BOOT_TIME_PAGE_SIZE
>>>> Kconfig, which is an alternative to selecting a compile-time page size.
>>>>
>>>> When boot-time page size is active, the arch pgtable geometry macro
>>>> definitions resolve to something that can be configured at boot. The arm64
>>>> implementation in this series mainly uses global, __ro_after_init
>>>> variables. I've tried using alternatives patching, but that performs worse
>>>> than loading from memory; I think due to code size bloat.
>>>>
>>>> Status
>>>> ======
>>>>
>>>> When CONFIG_ARM64_BOOT_TIME_PAGE_SIZE is selected, I've only implemented
>>>> enough to compile the kernel image itself with defconfig (and a few other
>>>> bits and pieces). This is enough to build a kernel that can boot under QEMU
>>>> or FVP. I'll happily do the rest of the work to enable all the extra
>>>> drivers, but wanted to get feedback on the shape of this effort first. If
>>>> anyone wants to do any testing, and has a must-have config, let me know and
>>>> I'll prioritize enabling it first.
>>>>
>>>> The series is arranged as follows:
>>>>
>>>>   - patch 1:    Add macros required for converting non-arch code to support
>>>>                 boot-time page size selection
>>>>   - patches 2-36:  Remove PAGE_SIZE compile-time constant assumption from
>>>> all non-arch code
>>>>   - patches 37-38: Some arm64 tidy ups
>>>>   - patch 39:           Add macros required for converting arm64 code to
>>> support
>>>>                 boot-time page size selection
>>>>   - patches 40-56: arm64 changes to support boot-time page size selection
>>>>   - patch 57:           Add arm64 Kconfig option to enable boot-time page
>>> size
>>>>                 selection
>>>>
>>>> Ideally, I'd like to get the basics merged (something like this series),
>>>> then incrementally improve it over a handful of kernel releases until we
>>>> can demonstrate that we have feature parity with the compile-time build and
>>>> no performance blockers. Once at that point, ideally the compile-time build
>>>> options would be removed and the code could be cleaned up further.
>>>>
>>>> One of the bigger peices that I'd propose to add as a follow up, is to make
>>>> va-size boot-time selectable too. That will greatly simplify LPA2 fallback
>>>> handling.
>>>>
>>>> Assuming people are ammenable to the rough shape, how would I go about
>>>> getting the non-arch changes merged? Since they cover many subsystems, will
>>>> each piece need to go independently to each relevant maintainer or could it
>>>> all be merged together through the arm64 tree?
>>>>
>>>> Image Size
>>>> ==========
>>>>
>>>> The below shows the size of a defconfig (+ xfs, squashfs, ftrace, kprobes)
>>>> kernel image on disk for base (before any changes applied), compile (with
>>>> changes, configured for compile-time page size) and boot (with changes,
>>>> configured for boot-time page size).
>>>>
>>>> You can see the that compile-16k and 64k configs are actually slightly
>>>> smaller than the baselines; that's due to optimizing some buffer sizes
>>>> which didn't need to depend on page size during the series. The boot-time
>>>> image is ~1% bigger than the 64k compile-time image. I believe there is
>>>> scope to improve this to make it
>>>> equal to compile-64k if required:
>>>> | config      | size/KB | diff/KB |  diff/% |
>>>> |
>>>> |-------------|---------|---------|---------|
>>>> |
>>>> | base-4k     |   54895 |       0 |    0.0% |
>>>> | base-16k    |   55161 |     266 |    0.5% |
>>>> | base-64k    |   56775 |    1880 |    3.4% |
>>>> | compile-4k  |   54895 |       0 |    0.0% |
>>>> | compile-16k |   55097 |     202 |    0.4% |
>>>> | compile-64k |   56391 |    1496 |    2.7% |
>>>> | boot-4K     |   57045 |    2150 |    3.9% |
>>>>
>>>> And below shows the size of the image in memory at run-time, separated for
>>>> text and data costs. The boot image has ~1% text cost; most likely due to
>>>> the fact that PAGE_SIZE and friends are not compile-time constants so need
>>>> instructions to load the values and do arithmetic. I believe we could
>>>> eventually get the data cost to match the cost for the compile image for
>>>> the chosen page size by freeing
>>>> the ends of the static buffers not needed for the selected page size:
>>>> |             |    text |    text |    text |    data |    data |    data |
>>>> |
>>>> | config      | size/KB | diff/KB |  diff/% | size/KB | diff/KB |  diff/% |
>>>> |
>>>> |-------------|---------|---------|---------|---------|---------|---------|
>>>> |
>>>> | base-4k     |   20561 |       0 |    0.0% |   14314 |       0 |    0.0% |
>>>> | base-16k    |   20439 |    -122 |   -0.6% |   14625 |     311 |    2.2% |
>>>> | base-64k    |   20435 |    -126 |   -0.6% |   15673 |    1359 |    9.5% |
>>>> | compile-4k  |   20565 |       4 |    0.0% |   14315 |       1 |    0.0% |
>>>> | compile-16k |   20443 |    -118 |   -0.6% |   14517 |     204 |    1.4% |
>>>> | compile-64k |   20439 |    -122 |   -0.6% |   15134 |     820 |    5.7% |
>>>> | boot-4K     |   20811 |     250 |    1.2% |   15287 |     973 |    6.8% |
>>>>
>>>> Functional Testing
>>>> ==================
>>>>
>>>> I've build-tested defconfig for all arches supported by tuxmake (which is
>>>> most) without issue.
>>>>
>>>> I've boot-tested arm64 with CONFIG_ARM64_BOOT_TIME_PAGE_SIZE for all page
>>>> sizes and a few va-sizes, and additionally have run all the mm-selftests,
>>>> with no regressions observed vs the equivalent compile-time page size build
>>>> (although the mm-selftests have a few existing failures when run against
>>>> 16K and 64K kernels - those should really be investigated and fixed
>>>> independently).
>>>>
>>>> Test coverage is lacking for many of the drivers that I've touched, but in
>>>> many cases, I'm hoping the changes are simple enough that review might
>>>> suffice?
>>>>
>>>> Performance Testing
>>>> ===================
>>>>
>>>> I've run some limited performance benchmarks:
>>>>
>>>> First, a real-world benchmark that causes a lot of page table manipulation
>>>> (and therefore we would expect to see regression here if we are going to
>>>> see it anywhere); kernel compilation. It barely registers a change. Values
>>>> are times,
>>>> so smaller is better. All relative to base-4k:
>>>> |             |    kern |    kern |    user |    user |    real |    real |
>>>> |
>>>> | config      |    mean |   stdev |    mean |   stdev |    mean |   stdev |
>>>> |
>>>> |-------------|---------|---------|---------|---------|---------|---------|
>>>> |
>>>> | base-4k     |    0.0% |    1.1% |    0.0% |    0.3% |    0.0% |    0.3% |
>>>> | compile-4k  |   -0.2% |    1.1% |   -0.2% |    0.3% |   -0.1% |    0.3% |
>>>> | boot-4k     |    0.1% |    1.0% |   -0.3% |    0.2% |   -0.2% |    0.2% |
>>>>
>>>> The Speedometer JavaScript benchmark also shows no change. Values are runs
>>>> per
>>>> min, so bigger is better. All relative to base-4k:
>>>> | config      |    mean |   stdev |
>>>> |
>>>> |-------------|---------|---------|
>>>> |
>>>> | base-4k     |    0.0% |    0.8% |
>>>> | compile-4k  |    0.4% |    0.8% |
>>>> | boot-4k     |    0.0% |    0.9% |
>>>>
>>>> Finally, I've run some microbenchmarks known to stress page table
>>>> manipulations (originally from David Hildenbrand). The fork test
>>>> maps/allocs 1G of anon memory, then measures the cost of fork(). The munmap
>>>> test maps/allocs 1G of anon memory then measures the cost of munmap()ing
>>>> it. The fork test is known to be extremely sensitive to any changes that
>>>> cause instructions to be aligned differently in cachelines. When using this
>>>> test for other changes, I've seen double digit regressions for the
>>>> slightest thing, so 12% regression on this test is actually fairly good.
>>>> This likely represents the extreme worst case for regressions that will be
>>>> observed across other microbenchmarks (famous last
>>>> words). Values are times, so smaller is better. All relative to base-4k:
>>>> |             |    fork |    fork |  munmap |  munmap |
>>>> |
>>>> | config      |    mean |   stdev |   stdev |   stdev |
>>>> |
>>>> |-------------|---------|---------|---------|---------|
>>>> |
>>>> | base-4k     |    0.0% |    1.3% |    0.0% |    0.3% |
>>>> | compile-4k  |    0.1% |    1.3% |   -0.9% |    0.1% |
>>>> | boot-4k     |   12.8% |    1.2% |    3.8% |    1.0% |
>>>>
>>>> NOTE: The series applies on top of v6.11.
>>>>
>>>> Thanks,
>>>> Ryan
>>>>
>>>>
>>>> Ryan Roberts (57):
>>>>   mm: Add macros ahead of supporting boot-time page size selection
>>>>   vmlinux: Align to PAGE_SIZE_MAX
>>>>   mm/memcontrol: Fix seq_buf size to save memory when PAGE_SIZE is large
>>>>   mm/page_alloc: Make page_frag_cache boot-time page size compatible
>>>>   mm: Avoid split pmd ptl if pmd level is run-time folded
>>>>   mm: Remove PAGE_SIZE compile-time constant assumption
>>>>   fs: Introduce MAX_BUF_PER_PAGE_SIZE_MAX for array sizing
>>>>   fs: Remove PAGE_SIZE compile-time constant assumption
>>>>   fs/nfs: Remove PAGE_SIZE compile-time constant assumption
>>>>   fs/ext4: Remove PAGE_SIZE compile-time constant assumption
>>>>   fork: Permit boot-time THREAD_SIZE determination
>>>>   cgroup: Remove PAGE_SIZE compile-time constant assumption
>>>>   bpf: Remove PAGE_SIZE compile-time constant assumption
>>>>   pm/hibernate: Remove PAGE_SIZE compile-time constant assumption
>>>>   stackdepot: Remove PAGE_SIZE compile-time constant assumption
>>>>   perf: Remove PAGE_SIZE compile-time constant assumption
>>>>   kvm: Remove PAGE_SIZE compile-time constant assumption
>>>>   trace: Remove PAGE_SIZE compile-time constant assumption
>>>>   crash: Remove PAGE_SIZE compile-time constant assumption
>>>>   crypto: Remove PAGE_SIZE compile-time constant assumption
>>>>   sunrpc: Remove PAGE_SIZE compile-time constant assumption
>>>>   sound: Remove PAGE_SIZE compile-time constant assumption
>>>>   net: Remove PAGE_SIZE compile-time constant assumption
>>>>   net: fec: Remove PAGE_SIZE compile-time constant assumption
>>>>   net: marvell: Remove PAGE_SIZE compile-time constant assumption
>>>>   net: hns3: Remove PAGE_SIZE compile-time constant assumption
>>>>   net: e1000: Remove PAGE_SIZE compile-time constant assumption
>>>>   net: igbvf: Remove PAGE_SIZE compile-time constant assumption
>>>>   net: igb: Remove PAGE_SIZE compile-time constant assumption
>>>>   drivers/base: Remove PAGE_SIZE compile-time constant assumption
>>>>   edac: Remove PAGE_SIZE compile-time constant assumption
>>>>   optee: Remove PAGE_SIZE compile-time constant assumption
>>>>   random: Remove PAGE_SIZE compile-time constant assumption
>>>>   sata_sil24: Remove PAGE_SIZE compile-time constant assumption
>>>>   virtio: Remove PAGE_SIZE compile-time constant assumption
>>>>   xen: Remove PAGE_SIZE compile-time constant assumption
>>>>   arm64: Fix macros to work in C code in addition to the linker script
>>>>   arm64: Track early pgtable allocation limit
>>>>   arm64: Introduce macros required for boot-time page selection
>>>>   arm64: Refactor early pgtable size calculation macros
>>>>   arm64: Pass desired page size on command line
>>>>   arm64: Divorce early init from PAGE_SIZE
>>>>   arm64: Clean up simple cases of CONFIG_ARM64_*K_PAGES
>>>>   arm64: Align sections to PAGE_SIZE_MAX
>>>>   arm64: Rework trampoline rodata mapping
>>>>   arm64: Generalize fixmap for boot-time page size
>>>>   arm64: Statically allocate and align for worst-case page size
>>>>   arm64: Convert switch to if for non-const comparison values
>>>>   arm64: Convert BUILD_BUG_ON to VM_BUG_ON
>>>>   arm64: Remove PAGE_SZ asm-offset
>>>>   arm64: Introduce cpu features for page sizes
>>>>   arm64: Remove PAGE_SIZE from assembly code
>>>>   arm64: Runtime-fold pmd level
>>>>   arm64: Support runtime folding in idmap_kpti_install_ng_mappings
>>>>   arm64: TRAMP_VALIAS is no longer compile-time constant
>>>>   arm64: Determine THREAD_SIZE at boot-time
>>>>   arm64: Enable boot-time page size selection
>>>>
>>>>  arch/alpha/include/asm/page.h                 |   1 +
>>>>  arch/arc/include/asm/page.h                   |   1 +
>>>>  arch/arm/include/asm/page.h                   |   1 +
>>>>  arch/arm64/Kconfig                            |  26 ++-
>>>>  arch/arm64/include/asm/assembler.h            |  78 ++++++-
>>>>  arch/arm64/include/asm/cpufeature.h           |  44 +++-
>>>>  arch/arm64/include/asm/efi.h                  |   2 +-
>>>>  arch/arm64/include/asm/fixmap.h               |  28 ++-
>>>>  arch/arm64/include/asm/kernel-pgtable.h       | 150 +++++++++----
>>>>  arch/arm64/include/asm/kvm_arm.h              |  21 +-
>>>>  arch/arm64/include/asm/kvm_hyp.h              |  11 +
>>>>  arch/arm64/include/asm/kvm_pgtable.h          |   6 +-
>>>>  arch/arm64/include/asm/memory.h               |  62 ++++--
>>>>  arch/arm64/include/asm/page-def.h             |   3 +-
>>>>  arch/arm64/include/asm/pgalloc.h              |  16 +-
>>>>  arch/arm64/include/asm/pgtable-geometry.h     |  46 ++++
>>>>  arch/arm64/include/asm/pgtable-hwdef.h        |  28 ++-
>>>>  arch/arm64/include/asm/pgtable-prot.h         |   2 +-
>>>>  arch/arm64/include/asm/pgtable.h              | 133 +++++++++---
>>>>  arch/arm64/include/asm/processor.h            |  10 +-
>>>>  arch/arm64/include/asm/sections.h             |   1 +
>>>>  arch/arm64/include/asm/smp.h                  |   1 +
>>>>  arch/arm64/include/asm/sparsemem.h            |  15 +-
>>>>  arch/arm64/include/asm/sysreg.h               |  54 +++--
>>>>  arch/arm64/include/asm/tlb.h                  |   3 +
>>>>  arch/arm64/kernel/asm-offsets.c               |   4 +-
>>>>  arch/arm64/kernel/cpufeature.c                |  93 ++++++--
>>>>  arch/arm64/kernel/efi.c                       |   2 +-
>>>>  arch/arm64/kernel/entry.S                     |  60 +++++-
>>>>  arch/arm64/kernel/head.S                      |  46 +++-
>>>>  arch/arm64/kernel/hibernate-asm.S             |   6 +-
>>>>  arch/arm64/kernel/image-vars.h                |  14 ++
>>>>  arch/arm64/kernel/image.h                     |   4 +
>>>>  arch/arm64/kernel/pi/idreg-override.c         |  68 +++++-
>>>>  arch/arm64/kernel/pi/map_kernel.c             | 165 ++++++++++----
>>>>  arch/arm64/kernel/pi/map_range.c              | 201 ++++++++++++++++--
>>>>  arch/arm64/kernel/pi/pi.h                     |  63 +++++-
>>>>  arch/arm64/kernel/relocate_kernel.S           |  10 +-
>>>>  arch/arm64/kernel/vdso-wrap.S                 |   4 +-
>>>>  arch/arm64/kernel/vdso.c                      |   7 +-
>>>>  arch/arm64/kernel/vdso/vdso.lds.S             |   4 +-
>>>>  arch/arm64/kernel/vdso32-wrap.S               |   4 +-
>>>>  arch/arm64/kernel/vdso32/vdso.lds.S           |   4 +-
>>>>  arch/arm64/kernel/vmlinux.lds.S               |  48 +++--
>>>>  arch/arm64/kvm/arm.c                          |  10 +
>>>>  arch/arm64/kvm/hyp/nvhe/Makefile              |   1 +
>>>>  arch/arm64/kvm/hyp/nvhe/host.S                |  10 +-
>>>>  arch/arm64/kvm/hyp/nvhe/hyp.lds.S             |   4 +-
>>>>  arch/arm64/kvm/hyp/nvhe/pgtable-geometry.c    |  16 ++
>>>>  arch/arm64/kvm/mmu.c                          |  39 ++--
>>>>  arch/arm64/lib/clear_page.S                   |   7 +-
>>>>  arch/arm64/lib/copy_page.S                    |  33 ++-
>>>>  arch/arm64/lib/mte.S                          |  27 ++-
>>>>  arch/arm64/mm/Makefile                        |   1 +
>>>>  arch/arm64/mm/fixmap.c                        |  38 ++--
>>>>  arch/arm64/mm/hugetlbpage.c                   |  40 +---
>>>>  arch/arm64/mm/init.c                          |  26 +--
>>>>  arch/arm64/mm/kasan_init.c                    |   8 +-
>>>>  arch/arm64/mm/mmu.c                           |  53 +++--
>>>>  arch/arm64/mm/pgd.c                           |  12 +-
>>>>  arch/arm64/mm/pgtable-geometry.c              |  24 +++
>>>>  arch/arm64/mm/proc.S                          | 128 ++++++++---
>>>>  arch/arm64/mm/ptdump.c                        |   3 +-
>>>>  arch/arm64/tools/cpucaps                      |   3 +
>>>>  arch/csky/include/asm/page.h                  |   3 +
>>>>  arch/hexagon/include/asm/page.h               |   2 +
>>>>  arch/loongarch/include/asm/page.h             |   2 +
>>>>  arch/m68k/include/asm/page.h                  |   1 +
>>>>  arch/microblaze/include/asm/page.h            |   1 +
>>>>  arch/mips/include/asm/page.h                  |   1 +
>>>>  arch/nios2/include/asm/page.h                 |   2 +
>>>>  arch/openrisc/include/asm/page.h              |   1 +
>>>>  arch/parisc/include/asm/page.h                |   1 +
>>>>  arch/powerpc/include/asm/page.h               |   2 +
>>>>  arch/riscv/include/asm/page.h                 |   1 +
>>>>  arch/s390/include/asm/page.h                  |   1 +
>>>>  arch/sh/include/asm/page.h                    |   1 +
>>>>  arch/sparc/include/asm/page.h                 |   3 +
>>>>  arch/um/include/asm/page.h                    |   2 +
>>>>  arch/x86/include/asm/page_types.h             |   2 +
>>>>  arch/xtensa/include/asm/page.h                |   1 +
>>>>  crypto/lskcipher.c                            |   4 +-
>>>>  drivers/ata/sata_sil24.c                      |  46 ++--
>>>>  drivers/base/node.c                           |   6 +-
>>>>  drivers/base/topology.c                       |  32 +--
>>>>  drivers/block/virtio_blk.c                    |   2 +-
>>>>  drivers/char/random.c                         |   4 +-
>>>>  drivers/edac/edac_mc.h                        |  13 +-
>>>>  drivers/firmware/efi/libstub/arm64.c          |   3 +-
>>>>  drivers/irqchip/irq-gic-v3-its.c              |   2 +-
>>>>  drivers/mtd/mtdswap.c                         |   4 +-
>>>>  drivers/net/ethernet/freescale/fec.h          |   3 +-
>>>>  drivers/net/ethernet/freescale/fec_main.c     |   5 +-
>>>>  .../net/ethernet/hisilicon/hns3/hns3_enet.h   |   4 +-
>>>>  drivers/net/ethernet/intel/e1000/e1000_main.c |   6 +-
>>>>  drivers/net/ethernet/intel/igb/igb.h          |  25 +--
>>>>  drivers/net/ethernet/intel/igb/igb_main.c     | 149 +++++++------
>>>>  drivers/net/ethernet/intel/igbvf/netdev.c     |   6 +-
>>>>  drivers/net/ethernet/marvell/mvneta.c         |   9 +-
>>>>  drivers/net/ethernet/marvell/sky2.h           |   2 +-
>>>>  drivers/tee/optee/call.c                      |   7 +-
>>>>  drivers/tee/optee/smc_abi.c                   |   2 +-
>>>>  drivers/virtio/virtio_balloon.c               |  10 +-
>>>>  drivers/xen/balloon.c                         |  11 +-
>>>>  drivers/xen/biomerge.c                        |  12 +-
>>>>  drivers/xen/privcmd.c                         |   2 +-
>>>>  drivers/xen/xenbus/xenbus_client.c            |   5 +-
>>>>  drivers/xen/xlate_mmu.c                       |   6 +-
>>>>  fs/binfmt_elf.c                               |  11 +-
>>>>  fs/buffer.c                                   |   2 +-
>>>>  fs/coredump.c                                 |   8 +-
>>>>  fs/ext4/ext4.h                                |  36 ++--
>>>>  fs/ext4/move_extent.c                         |   2 +-
>>>>  fs/ext4/readpage.c                            |   2 +-
>>>>  fs/fat/dir.c                                  |   4 +-
>>>>  fs/fat/fatent.c                               |   4 +-
>>>>  fs/nfs/nfs42proc.c                            |   2 +-
>>>>  fs/nfs/nfs42xattr.c                           |   2 +-
>>>>  fs/nfs/nfs4proc.c                             |   2 +-
>>>>  include/asm-generic/pgtable-geometry.h        |  71 +++++++
>>>>  include/asm-generic/vmlinux.lds.h             |  38 ++--
>>>>  include/linux/buffer_head.h                   |   1 +
>>>>  include/linux/cpumask.h                       |   5 +
>>>>  include/linux/linkage.h                       |   4 +-
>>>>  include/linux/mm.h                            |  17 +-
>>>>  include/linux/mm_types.h                      |  15 +-
>>>>  include/linux/mm_types_task.h                 |   2 +-
>>>>  include/linux/mmzone.h                        |   3 +-
>>>>  include/linux/netlink.h                       |   6 +-
>>>>  include/linux/percpu-defs.h                   |   4 +-
>>>>  include/linux/perf_event.h                    |   2 +-
>>>>  include/linux/sched.h                         |   4 +-
>>>>  include/linux/slab.h                          |   7 +-
>>>>  include/linux/stackdepot.h                    |   6 +-
>>>>  include/linux/sunrpc/svc.h                    |   8 +-
>>>>  include/linux/sunrpc/svc_rdma.h               |   4 +-
>>>>  include/linux/sunrpc/svcsock.h                |   2 +-
>>>>  include/linux/swap.h                          |  17 +-
>>>>  include/linux/swapops.h                       |   6 +-
>>>>  include/linux/thread_info.h                   |  10 +-
>>>>  include/xen/page.h                            |   2 +
>>>>  init/main.c                                   |   7 +-
>>>>  kernel/bpf/core.c                             |   9 +-
>>>>  kernel/bpf/ringbuf.c                          |  54 ++---
>>>>  kernel/cgroup/cgroup.c                        |   8 +-
>>>>  kernel/crash_core.c                           |   2 +-
>>>>  kernel/events/core.c                          |   2 +-
>>>>  kernel/fork.c                                 |  71 +++----
>>>>  kernel/power/power.h                          |   2 +-
>>>>  kernel/power/snapshot.c                       |   2 +-
>>>>  kernel/power/swap.c                           | 129 +++++++++--
>>>>  kernel/trace/fgraph.c                         |   2 +-
>>>>  kernel/trace/trace.c                          |   2 +-
>>>>  lib/stackdepot.c                              |   6 +-
>>>>  mm/kasan/report.c                             |   3 +-
>>>>  mm/memcontrol.c                               |  11 +-
>>>>  mm/memory.c                                   |   4 +-
>>>>  mm/mmap.c                                     |   2 +-
>>>>  mm/page-writeback.c                           |   2 +-
>>>>  mm/page_alloc.c                               |  31 +--
>>>>  mm/slub.c                                     |   2 +-
>>>>  mm/sparse.c                                   |   2 +-
>>>>  mm/swapfile.c                                 |   2 +-
>>>>  mm/vmalloc.c                                  |   7 +-
>>>>  net/9p/trans_virtio.c                         |   4 +-
>>>>  net/core/hotdata.c                            |   4 +-
>>>>  net/core/skbuff.c                             |   4 +-
>>>>  net/core/sysctl_net_core.c                    |   2 +-
>>>>  net/sunrpc/cache.c                            |   3 +-
>>>>  net/unix/af_unix.c                            |   2 +-
>>>>  sound/soc/soc-utils.c                         |   4 +-
>>>>  virt/kvm/kvm_main.c                           |   2 +-
>>>>  172 files changed, 2185 insertions(+), 951 deletions(-)
>>>>  create mode 100644 arch/arm64/include/asm/pgtable-geometry.h
>>>>  create mode 100644 arch/arm64/kvm/hyp/nvhe/pgtable-geometry.c
>>>>  create mode 100644 arch/arm64/mm/pgtable-geometry.c
>>>>  create mode 100644 include/asm-generic/pgtable-geometry.h
>>>>
>>>> --
>>>> 2.43.0
>>>
>>> This is a generally very exciting patch set! I'm looking forward to seeing it
>>> land so I can take advantage of it for Fedora ARM and Fedora Asahi Remix.
>>>
>>> That said, I have a couple of questions:
>>>
>>> * Going forward, how would we handle drivers/modules that require a particular
>>> page size? For example, the Apple Silicon IOMMU driver code requires the
>>> kernel to operate in 16k page size mode, and it would need to be disabled in
>>> other page sizes.
>>
>> I think these drivers would want to check PAGE_SIZE at probe time and fail if an
>> unsupported page size is in use. Do you see any issue with that?
>>
>>>
>>> * How would we handle an invalid selection at boot?
>>
>> What do you mean by invalid here? The current policy validates that the
>> requested page size is supported by the HW by checking mmfr0. If no page size is
>> passed on the command line, or the passed value is not supported by the HW, then
>> the we default to the largest page size supported by the HW (so for Apple
>> Silicon that would be 16k since the HW doesn't support 64k). Although I think it
>> may be better to change that policy to use the smallest page size in this case;
>> 4k is the safer bet for compat and will waste much less memory than 64k.
>>
>>> Can we program in a
>>> fallback when the "wrong" mode is selected for a chip or something similar?
>>
>> Do you mean effectively add a machanism to force 16k if the detected HW is Apple
>> Silicon? The trouble is that we need to select the page size, very early in
>> boot, before start_kernel() is called, so we really only have generic arch code
>> and the command line with which to make the decision.
> 
> Yes... I think a build-time CONFIG for default page size, which can be
> overridden by a karg makes sense... Even on platforms like Apple
> Silicon you may want to test very specific things in 4k by overriding
> with a karg.

Ahh, yes, that would certainly work. I'll work it into the next version.

> 
> Like in downstream kernels like Fedora/RHEL/etc. I would expect the
> default would be 4k, but you could override with 16k, 64k, etc. with a
> karg.
> 
>>
>>>> Thanks again and best regards!
>>>
>>> (P.S.: Please add the asahi@ mailing list to the CC for future iterations of
>>> this patch set and tag both Hector and myself in as well. Thanks!)
>>
>> Will do!
>>
>>>
>>>
>>
>>
>