Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically split/reconstruct HugeTLB page

[Yan Zhao <yan.y.zhao@intel.com>]

On Tue, May 06, 2025 at 12:22:47PM -0700, Ackerley Tng wrote:
> Yan Zhao <yan.y.zhao@intel.com> writes:
> 
> >> > <snip>
> >> >
> >> > What options does userspace have in this scenario?
> >> > It can't reduce the flag to KVM_GUEST_MEMFD_HUGE_2MB. Adjusting the gmem.pgoff
> >> > isn't ideal either.
> >> >
> >> > What about something similar as below?
> >> >
> >> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> >> > index d2feacd14786..87c33704a748 100644
> >> > --- a/virt/kvm/guest_memfd.c
> >> > +++ b/virt/kvm/guest_memfd.c
> >> > @@ -1842,8 +1842,16 @@ __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
> >> >         }
> >> >
> >> >         *pfn = folio_file_pfn(folio, index);
> >> > -       if (max_order)
> >> > -               *max_order = folio_order(folio);
> >> > +       if (max_order) {
> >> > +               int order;
> >> > +
> >> > +               order = folio_order(folio);
> >> > +
> >> > +               while (order > 0 && ((slot->base_gfn ^ slot->gmem.pgoff) & ((1 << order) - 1)))
> >> > +                       order--;
> >> > +
> >> > +               *max_order = order;
> >> > +       }
> >> >
> >> >         *is_prepared = folio_test_uptodate(folio);
> >> >         return folio;
> >> >
> >> 
> >> Vishal was wondering how this is working before guest_memfd was
> >> introduced, for other backing memory like HugeTLB.
> >> 
> >> I then poked around and found this [1]. I will be adding a similar check
> >> for any slot where kvm_slot_can_be_private(slot).
> >>
> >> Yan, that should work, right?
> > No, I don't think the checking of ugfn [1] should work.
> >
> > 1. Even for slots bound to in-place-conversion guest_memfd (i.e. shared memory
> > are allocated from guest_memfd), the slot->userspace_addr does not necessarily
> > have the same offset as slot->gmem.pgoff. Even if we audit the offset in
> > kvm_gmem_bind(), userspace could invoke munmap() and mmap() afterwards, causing
> > slot->userspace_addr to point to a different offset.
> >
> > 2. for slots bound to guest_memfd that do not support in-place-conversion,
> > shared memory is allocated from a different backend. Therefore, checking
> > "slot->base_gfn ^ slot->gmem.pgoff" is required for private memory. The check is
> > currently absent because guest_memfd supports 4K only.
> >
> >
> 
> Let me clarify, I meant these changes:
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4b64ab3..d0dccf1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12938,6 +12938,11 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages)
>         return 0;
>  }
>  
> +static inline bool kvm_is_level_aligned(u64 value, int level)
> +{
> +       return IS_ALIGNED(value, KVM_PAGES_PER_HPAGE(level));
> +}
> +
>  static int kvm_alloc_memslot_metadata(struct kvm *kvm,
>                                       struct kvm_memory_slot *slot)
>  {
> @@ -12971,16 +12976,20 @@ static int kvm_alloc_memslot_metadata(struct kvm *kvm,
>  
>                 slot->arch.lpage_info[i - 1] = linfo;
>  
> -               if (slot->base_gfn & (KVM_PAGES_PER_HPAGE(level) - 1))
> +               if (!kvm_is_level_aligned(slot->base_gfn, level))
>                         linfo[0].disallow_lpage = 1;
> -               if ((slot->base_gfn + npages) & (KVM_PAGES_PER_HPAGE(level) - 1))
> +               if (!kvm_is_level_aligned(slot->base_gfn + npages, level))
>                         linfo[lpages - 1].disallow_lpage = 1;
>                 ugfn = slot->userspace_addr >> PAGE_SHIFT;
>                 /*
> -                * If the gfn and userspace address are not aligned wrt each
> -                * other, disable large page support for this slot.
> +                * If the gfn and userspace address are not aligned or if gfn
> +                * and guest_memfd offset are not aligned wrt each other,
> +                * disable large page support for this slot.
>                  */
> -               if ((slot->base_gfn ^ ugfn) & (KVM_PAGES_PER_HPAGE(level) - 1)) {
> +               if (!kvm_is_level_aligned(slot->base_gfn ^ ugfn, level) ||
> +                   (kvm_slot_can_be_private(slot) &&
> +                    !kvm_is_level_aligned(slot->base_gfn ^ slot->gmem.pgoff,
> +                                          level))) {
>                         unsigned long j;
>  
>                         for (j = 0; j < lpages; ++j)
> 
> This does not rely on the ugfn check, but adds a similar check for gmem.pgoff.
In the case of shared memory is not allocated from guest_memfd, (e.g. with the
current upstream code), the checking of gmem.pgoff here will disallow huge page
of shared memory even if "slot->base_gfn ^ ugfn" is aligned.

> I think this should take care of case (1.), for guest_memfds going to be
> used for both shared and private memory. Userspace can't update
> slot->userspace_addr, since guest_memfd memslots cannot be updated and
> can only be deleted.
> 
> If userspace re-uses slot->userspace_addr for some other memory address
> without deleting and re-adding a memslot,
> 
> + KVM's access to memory should still be fine, since after the recent
>   discussion at guest_memfd upstream call, KVM's guest faults will
>   always go via fd+offset and KVM's access won't be disrupted
>   there. Whatever checking done at memslot binding time will still be
>   valid.
Could the offset of shared memory and offset of private memory be different if
userspace re-uses slot->userspace_addr without deleting and re-adding a memslot?

Then though the two offsets are validated as equal in kvm_gmem_bind(), they may
differ later on.

> + Host's access and other accesses (e.g. instruction emulation, which
>   uses slot->userspace_addr) to guest memory will be broken, but I think
>   there's nothing protecting against that. The same breakage would
>   happen for non-guest_memfd memslot.
Why is host access broken in non-guest_memfd case?
The HVA is still a valid one in QEMU's mmap-ed address space.

> p.s. I will be adding the validation as you suggested [1], though that
> shouldn't make a difference here, since the above check directly
> validates against gmem.pgoff.
> 
> Regarding 2., checking this checks against gmem.pgoff and should handle
> that as well.
> 
> [1] https://lore.kernel.org/all/aBnMp26iWWhUrsVf@yzhao56-desk.sh.intel.com/
> 
> I prefer checking at binding time because it aligns with the ugfn check
> that is already there, and avoids having to check at every fault.
> 
> >> [1] https://github.com/torvalds/linux/blob/b6ea1680d0ac0e45157a819c41b46565f4616186/arch/x86/kvm/x86.c#L12996
> >> 
> >> >> >> Adding checks at binding time will allow hugepage-unaligned offsets (to
> >> >> >> be at parity with non-guest_memfd backing memory) but still fix this
> >> >> >> issue.
> >> >> >> 
> >> >> >> lpage_info will make sure that ranges near the bounds will be
> >> >> >> fragmented, but the hugepages in the middle will still be mappable as
> >> >> >> hugepages.
> >> >> >> 
> >> >> >> [1] https://lpc.events/event/18/contributions/1764/attachments/1409/3706/binding-must-have-same-alignment.svg