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Date:   Wed, 1 Nov 2023 09:36:00 -0700
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Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for
 dedicated guest memory
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.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>,
        Anish Moorthy <amoorthy@google.com>,
        David Matlack <dmatlack@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
        "=?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?=" <mic@digikod.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        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 01, 2023, Paolo Bonzini wrote:
> On Wed, Nov 1, 2023 at 2:41=E2=80=AFPM Sean Christopherson <seanjc@google=
.com> wrote:
> >
> > On Wed, Nov 01, 2023, Xiaoyao Li wrote:
> > > On 10/31/2023 10:16 PM, Sean Christopherson wrote:
> > > > On Tue, Oct 31, 2023, Xiaoyao Li wrote:
> > > > > On 10/28/2023 2:21 AM, Sean Christopherson wrote:
> > > But it's different than MADV_HUGEPAGE, in a way. Per my understanding=
, the
> > > failure of MADV_HUGEPAGE is not fatal, user space can ignore it and
> > > continue.
> > >
> > > However, the failure of KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is fatal, whic=
h leads
> > > to failure of guest memfd creation.
> >
> > Failing KVM_CREATE_GUEST_MEMFD isn't truly fatal, it just requires diff=
erent
> > action from userspace, i.e. instead of ignoring the error, userspace co=
uld redo
> > KVM_CREATE_GUEST_MEMFD with KVM_GUEST_MEMFD_ALLOW_HUGEPAGE=3D0.
> >
> > We could make the behavior more like MADV_HUGEPAGE, e.g. theoretically =
we could
> > extend fadvise() with FADV_HUGEPAGE, or add a guest_memfd knob/ioctl() =
to let
> > userspace provide advice/hints after creating a guest_memfd.  But I sus=
pect that
> > guest_memfd would be the only user of FADV_HUGEPAGE, and IMO a post-cre=
ation hint
> > is actually less desirable.
> >
> > KVM_GUEST_MEMFD_ALLOW_HUGEPAGE will fail only if userspace didn't provi=
de a
> > compatible size or the kernel doesn't support THP.  An incompatible siz=
e is likely
> > a userspace bug, and for most setups that want to utilize guest_memfd, =
lack of THP
> > support is likely a configuration bug.  I.e. many/most uses *want* fail=
ures due to
> > KVM_GUEST_MEMFD_ALLOW_HUGEPAGE to be fatal.
> >
> > > For current implementation, I think maybe KVM_GUEST_MEMFD_DESIRE_HUGE=
PAGE
> > > fits better than KVM_GUEST_MEMFD_ALLOW_HUGEPAGE? or maybe *PREFER*?
> >
> > Why?  Verbs like "prefer" and "desire" aren't a good fit IMO because th=
ey suggest
> > the flag is a hint, and hints are usually best effort only, i.e. are ig=
nored if
> > there is a fundamental incompatibility.
> >
> > "Allow" isn't perfect, e.g. I would much prefer a straight KVM_GUEST_ME=
MFD_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 stro=
nger than
> > a hint, but weaker than a requirement.  And if/when KVM supports a dedi=
cated memory
> > pool of some kind, then we can add KVM_GUEST_MEMFD_REQUIRE_HUGEPAGE.
>=20
> 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_HU=
GEPAGE.
> 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 i=
s 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.