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SubjectRe: [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory
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On 11/1/23 17:36, Sean Christopherson wrote:
>>> "Allow" isn't perfect, e.g. I would much prefer a straight KVM_GUEST_MEMFD_USE_HUGEPAGES
>>> or KVM_GUEST_MEMFD_HUGEPAGES flag, but I wanted the name to convey that KVM doesn't
>>> (yet) guarantee hugepages. I.e. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is stronger than
>>> a hint, but weaker than a requirement. And if/when KVM supports a dedicated memory
>>> pool of some kind, then we can add KVM_GUEST_MEMFD_REQUIRE_HUGEPAGE.
>> I think that the current patch is fine, but I will adjust it to always
>> allow the flag, and to make the size check even if !CONFIG_TRANSPARENT_HUGEPAGE.
>> If hugepages are not guaranteed, and (theoretically) you could have no
>> hugepage at all in the result, it's okay to get this result even if THP is not
>> available in the kernel.
> Can you post a fixup patch? It's not clear to me exactly what behavior you intend
> to end up with.

Sure, just this:

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 7d1a33c2ad42..34fd070e03d9 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -430,10 +430,7 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
{
loff_t size = args->size;
u64 flags = args->flags;
- u64 valid_flags = 0;
-
- if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
- valid_flags |= KVM_GUEST_MEMFD_ALLOW_HUGEPAGE;
+ u64 valid_flags = KVM_GUEST_MEMFD_ALLOW_HUGEPAGE;

if (flags & ~valid_flags)
return -EINVAL;
@@ -441,11 +438,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
if (size < 0 || !PAGE_ALIGNED(size))
return -EINVAL;

-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
if ((flags & KVM_GUEST_MEMFD_ALLOW_HUGEPAGE) &&
!IS_ALIGNED(size, HPAGE_PMD_SIZE))
return -EINVAL;
-#endif

return __kvm_gmem_create(kvm, size, flags);
}
Paolo

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