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From:   Paolo Bonzini <pbonzini@redhat.com>
Date:   Thu, 2 Nov 2023 00:17:13 +0100
Message-ID: <CABgObfbq_Hg0B=jvsSDqYH3CSpX+RsxfwB-Tc-eYF4uq2Qw9cg@mail.gmail.com>
Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for
 dedicated guest memory
To:     Sean Christopherson <seanjc@google.com>
Cc:     Xiaoyao Li <xiaoyao.li@intel.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Xu Yilun <yilun.xu@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
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        Ackerley Tng <ackerleytng@google.com>,
        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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On Wed, Nov 1, 2023 at 11:35=E2=80=AFPM Sean Christopherson <seanjc@google.=
com> 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_GU=
EST_MEMFD_USE_HUGEPAGES
> > > > > or KVM_GUEST_MEMFD_HUGEPAGES flag, but I wanted the name to conve=
y that KVM doesn't
> > > > > (yet) guarantee hugepages.  I.e. KVM_GUEST_MEMFD_ALLOW_HUGEPAGE i=
s 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_HUGEPA=
GE.
> > > > I think that the current patch is fine, but I will adjust it to alw=
ays
> > > > allow the flag, and to make the size check even if !CONFIG_TRANSPAR=
ENT_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 behavi=
or 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_cr=
eate_guest_memfd *args)
> >  {
> >       loff_t size =3D args->size;
> >       u64 flags =3D args->flags;
> > -     u64 valid_flags =3D 0;
> > -
> > -     if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> > -             valid_flags |=3D KVM_GUEST_MEMFD_ALLOW_HUGEPAGE;
> > +     u64 valid_flags =3D KVM_GUEST_MEMFD_ALLOW_HUGEPAGE;
> >       if (flags & ~valid_flags)
> >               return -EINVAL;
> > @@ -441,11 +438,9 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_cr=
eate_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_HUGE=
PAGE=3Dy.
>
> #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 !=3D PMD_SIZE);
#endif

for extra safety.

Paolo