Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory

[Paolo Bonzini]

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