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

[Paolo Bonzini]

On Wed, Nov 1, 2023 at 11:35 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Nov 01, 2023, Paolo Bonzini wrote:
> > 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
>
> That won't work, HPAGE_PMD_SIZE is valid only for CONFIG_TRANSPARENT_HUGEPAGE=y.
>
> #else /* CONFIG_TRANSPARENT_HUGEPAGE */
> #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })
> #define HPAGE_PMD_MASK ({ BUILD_BUG(); 0; })
> #define HPAGE_PMD_SIZE ({ BUILD_BUG(); 0; })

Would have caught it when actually testing it, I guess. :) It has to
be PMD_SIZE, possibly with

#ifdef CONFIG_TRANSPARENT_HUGEPAGE
BUILD_BUG_ON(HPAGE_PMD_SIZE != PMD_SIZE);
#endif

for extra safety.

Paolo