Re: [RFC PATCH v1 2/9] KVM: guest_memfd: Add guest_memfd support to kvm_(read|/write)_guest_page()

[Patrick Roy <roypat@amazon.co.uk>]

On Thu, 2025-01-23 at 11:39 +0000, David Hildenbrand wrote:
> On 23.01.25 10:48, Fuad Tabba wrote:
>> On Wed, 22 Jan 2025 at 22:10, David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 22.01.25 16:27, Fuad Tabba wrote:
>>>> Make kvm_(read|/write)_guest_page() capable of accessing guest
>>>> memory for slots that don't have a userspace address, but only if
>>>> the memory is mappable, which also indicates that it is
>>>> accessible by the host.
>>>
>>> Interesting. So far my assumption was that, for shared memory, user
>>> space would simply mmap() guest_memdd and pass it as userspace address
>>> to the same memslot that has this guest_memfd for private memory.
>>>
>>> Wouldn't that be easier in the first shot? (IOW, not require this patch
>>> with the cost of faulting the shared page into the page table on access)
>>
> 
> In light of:
> 
> https://lkml.kernel.org/r/20250117190938.93793-4-imbrenda@linux.ibm.com
> 
> there can, in theory, be memslots that start at address 0 and have a
> "valid" mapping. This case is done from the kernel (and on special s390x
> hardware), though, so it does not apply here at all so far.
> 
> In practice, getting address 0 as a valid address is unlikely, because
> the default:
> 
> $ sysctl  vm.mmap_min_addr
> vm.mmap_min_addr = 65536
> 
> usually prohibits it for good reason.
> 
>> This has to do more with the ABI I had for pkvm and shared memory
>> implementations, in which you don't need to specify the userspace
>> address for memory in a guestmem memslot. The issue is there is no
>> obvious address to map it to. This would be the case in kvm:arm64 for
>> tracking paravirtualized time, which the userspace doesn't necessarily
>> need to interact with, but kvm does.
> 
> So I understand correctly: userspace wouldn't have to mmap it because it
> is not interested in accessing it, but there is nothing speaking against
> mmaping it, at least in the first shot.
> 
> I assume it would not be a private memslot (so far, my understanding is
> that internal memslots never have a guest_memfd attached).
> kvm_gmem_create() is only called via KVM_CREATE_GUEST_MEMFD, to be set
> on user-created memslots.
> 
>>
>> That said, we could always have a userspace address dedicated to
>> mapping shared locations, and use that address when the necessity
>> arises. Or we could always require that memslots have a userspace
>> address, even if not used. I don't really have a strong preference.
> 
> So, the simpler version where user space would simply mmap guest_memfd
> to provide the address via userspace_addr would at least work for the
> use case of paravirtualized time?

fwiw, I'm currently prototyping something like this for x86 (although
not by putting the gmem address into userspace_addr, but by adding a new
field to memslots, so that memory attributes continue working), based on
what we talked about at the last guest_memfd sync meeting (the whole
"how to get MMIO emulation working for non-CoCo VMs in guest_memfd"
story). So I guess if we're going down this route for x86, maybe it
makes sense to do the same on ARM, for consistency?

> It would get rid of the immediate need for this patch and patch #4 to
> get it flying.
> 
> 
> One interesting question is: when would you want shared memory in
> guest_memfd and *not* provide it as part of the same memslot. 

In my testing of non-CoCo gmem VMs on ARM, I've been able to get quite
far without giving KVM a way to internally access shared parts of gmem - 
it's why I was probing Fuad for this simplified series, because
KVM_SW_PROTECTED_VM + mmap (for loading guest kernel) is enough to get a
working non-CoCo VM on ARM (although I admittedly never looked at clocks
inside the guest - maybe that's one thing that breaks if KVM can't
access gmem. How to guest and host agree on the guest memory range
used to exchange paravirtual timekeeping information? Could that exchange
be intercepted in userspace, and set to shared via memory attributes (e.g.
placed outside gmem)? That's the route I'm going down the paravirtual
time on x86).

> One nice thing about the mmap might be that access go via user-space
> page tables: E.g., __kvm_read_guest_page can just access the memory
> without requiring the folio lock and an additional temporary folio
> reference on every access -- it's handled implicitly via the mapcount.
> 
> (of course, to map the page we still need that once on the fault path)

Doing a direct map access in kvm_{read,write}_guest() and friends will
also get tricky if guest_memfd folios ever don't have direct map
entries. On-demand restoration is painful, both complexity and
performance wise [1], while going through a userspace mapping of
guest_memfd would "just work".

> -- 
> Cheers,
> 
> David / dhildenb
>