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

Fuad Tabba
2025-01-23T12:29:20Z

Hi Patrick,

On Thu, 23 Jan 2025 at 11:57, Patrick Roy <roypat@amazon.co.uk> wrote:
>
>
>
> 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).

For an idea of what it looks like on arm64, here's how kvmtool handles it:
https://github.com/kvmtool/kvmtool/blob/master/arm/aarch64/pvtime.c

Cheers,
/fuad

> > 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
> >