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

Ackerley Tng
2025-05-06T19:22:49Z

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.

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.
+ 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.

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