Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Anshuman,

On Wed, Mar 2, 2022 at 10:51 AM Anshuman Khandual
<anshuman.khandual@arm.com> wrote:
> On 3/2/22 12:35 PM, Christophe Leroy wrote:
> > Le 02/03/2022 à 04:22, Anshuman Khandual a écrit :
> >> On 3/1/22 1:46 PM, Christophe Leroy wrote:
> >>> Le 01/03/2022 à 01:31, Russell King (Oracle) a écrit :
> >>>> On Tue, Mar 01, 2022 at 05:30:41AM +0530, Anshuman Khandual wrote:
> >>>>> On 2/28/22 4:27 PM, Russell King (Oracle) wrote:
> >>>>>> On Mon, Feb 28, 2022 at 04:17:32PM +0530, Anshuman Khandual wrote:
> >>>>>>> This defines and exports a platform specific custom vm_get_page_prot() via
> >>>>>>> subscribing ARCH_HAS_VM_GET_PAGE_PROT. Subsequently all __SXXX and __PXXX
> >>>>>>> macros can be dropped which are no longer needed.
> >>>>>>
> >>>>>> What I would really like to know is why having to run _code_ to work out
> >>>>>> what the page protections need to be is better than looking it up in a
> >>>>>> table.
> >>>>>>
> >>>>>> Not only is this more expensive in terms of CPU cycles, it also brings
> >>>>>> additional code size with it.
> >>>>>>
> >>>>>> I'm struggling to see what the benefit is.
> >>>>>
> >>>>> Currently vm_get_page_prot() is also being _run_ to fetch required page
> >>>>> protection values. Although that is being run in the core MM and from a
> >>>>> platform perspective __SXXX, __PXXX are just being exported for a table.
> >>>>> Looking it up in a table (and applying more constructs there after) is
> >>>>> not much different than a clean switch case statement in terms of CPU
> >>>>> usage. So this is not more expensive in terms of CPU cycles.
> >>>>
> >>>> I disagree.
> >>>
> >>> So do I.
> >>>
> >>>>
> >>>> However, let's base this disagreement on some evidence. Here is the
> >>>> present 32-bit ARM implementation:
> >>>>
> >>>> 00000048 <vm_get_page_prot>:
> >>>>         48:       e200000f        and     r0, r0, #15
> >>>>         4c:       e3003000        movw    r3, #0
> >>>>                           4c: R_ARM_MOVW_ABS_NC   .LANCHOR1
> >>>>         50:       e3403000        movt    r3, #0
> >>>>                           50: R_ARM_MOVT_ABS      .LANCHOR1
> >>>>         54:       e7930100        ldr     r0, [r3, r0, lsl #2]
> >>>>         58:       e12fff1e        bx      lr
> >>>>
> >>>> That is five instructions long.
> >>>
> >>> On ppc32 I get:
> >>>
> >>> 00000094 <vm_get_page_prot>:
> >>>         94: 3d 20 00 00     lis     r9,0
> >>>                     96: R_PPC_ADDR16_HA     .data..ro_after_init
> >>>         98: 54 84 16 ba     rlwinm  r4,r4,2,26,29
> >>>         9c: 39 29 00 00     addi    r9,r9,0
> >>>                     9e: R_PPC_ADDR16_LO     .data..ro_after_init
> >>>         a0: 7d 29 20 2e     lwzx    r9,r9,r4
> >>>         a4: 91 23 00 00     stw     r9,0(r3)
> >>>         a8: 4e 80 00 20     blr
> >>>
> >>>
> >>>>
> >>>> Please show that your new implementation is not more expensive on
> >>>> 32-bit ARM. Please do so by building a 32-bit kernel, and providing
> >>>> the disassembly.
> >>>
> >>> With your series I get:
> >>>
> >>> 00000000 <vm_get_page_prot>:
> >>>      0:     3d 20 00 00     lis     r9,0
> >>>                     2: R_PPC_ADDR16_HA      .rodata
> >>>      4:     39 29 00 00     addi    r9,r9,0
> >>>                     6: R_PPC_ADDR16_LO      .rodata
> >>>      8:     54 84 16 ba     rlwinm  r4,r4,2,26,29
> >>>      c:     7d 49 20 2e     lwzx    r10,r9,r4
> >>>     10:     7d 4a 4a 14     add     r10,r10,r9
> >>>     14:     7d 49 03 a6     mtctr   r10
> >>>     18:     4e 80 04 20     bctr
> >>>     1c:     39 20 03 15     li      r9,789
> >>>     20:     91 23 00 00     stw     r9,0(r3)
> >>>     24:     4e 80 00 20     blr
> >>>     28:     39 20 01 15     li      r9,277
> >>>     2c:     91 23 00 00     stw     r9,0(r3)
> >>>     30:     4e 80 00 20     blr
> >>>     34:     39 20 07 15     li      r9,1813
> >>>     38:     91 23 00 00     stw     r9,0(r3)
> >>>     3c:     4e 80 00 20     blr
> >>>     40:     39 20 05 15     li      r9,1301
> >>>     44:     91 23 00 00     stw     r9,0(r3)
> >>>     48:     4e 80 00 20     blr
> >>>     4c:     39 20 01 11     li      r9,273
> >>>     50:     4b ff ff d0     b       20 <vm_get_page_prot+0x20>
> >>>
> >>>
> >>> That is definitely more expensive, it implements a table of branches.
> >>
> >> Okay, will split out the PPC32 implementation that retains existing
> >> table look up method. Also planning to keep that inside same file
> >> (arch/powerpc/mm/mmap.c), unless you have a difference preference.
> >
> > My point was not to get something specific for PPC32, but to amplify on
> > Russell's objection.
> >
> > As this is bad for ARM and bad for PPC32, do we have any evidence that
> > your change is good for any other architecture ?
> >
> > I checked PPC64 and there is exactly the same drawback. With the current
> > implementation it is a small function performing table read then a few
> > adjustment. After your change it is a bigger function implementing a
> > table of branches.
>
> I am wondering if this would not be the case for any other switch case
> statement on the platform ? Is there something specific/different just
> on vm_get_page_prot() implementation ? Are you suggesting that switch
> case statements should just be avoided instead ?
>
> >
> > So, as requested by Russell, could you look at the disassembly for other
> > architectures and show us that ARM and POWERPC are the only ones for
> > which your change is not optimal ?
>
> But the primary purpose of this series is not to guarantee optimized
> code on platform by platform basis, while migrating from a table based
> look up method into a switch case statement.
>
> But instead, the purposes is to remove current levels of unnecessary
> abstraction while converting a vm_flags access combination into page
> protection. The switch case statement for platform implementation of
> vm_get_page_prot() just seemed logical enough. Christoph's original
> suggestion patch for x86 had the same implementation as well.
>
> But if the table look up is still better/preferred method on certain
> platforms like arm or ppc32, will be happy to preserve that.

I doubt the switch() variant would give better code on any platform.

What about using tables everywhere, using designated initializers
to improve readability?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds