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From:   Fuad Tabba <tabba@google.com>
Date:   Thu, 2 Nov 2023 14:57:10 +0000
Message-ID: <CA+EHjTwpVSyUC8oiii3EYU8pqEJ0D0NXA1EZt76wjfPBpk2rFA@mail.gmail.com>
Subject: Re: [PATCH v13 12/35] KVM: Prepare for handling only shared mappings
 in mmu_notifier events
To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.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, Xiaoyao Li <xiaoyao.li@intel.com>,
        Xu Yilun <yilun.xu@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.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?B?TWlja2HDq2wgU2FsYcO8bg==?= <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 Thu, Nov 2, 2023 at 2:41=E2=80=AFPM Sean Christopherson <seanjc@google.c=
om> wrote:
>
> On Thu, Nov 02, 2023, Fuad Tabba wrote:
> > Hi,
> >
> > On Fri, Oct 27, 2023 at 7:22=E2=80=AFPM Sean Christopherson <seanjc@goo=
gle.com> wrote:
> > >
> > > Add flags to "struct kvm_gfn_range" to let notifier events target onl=
y
> > > shared and only private mappings, and write up the existing mmu_notif=
ier
> > > events to be shared-only (private memory is never associated with a
> > > userspace virtual address, i.e. can't be reached via mmu_notifiers).
> > >
> > > Add two flags so that KVM can handle the three possibilities (shared,
> > > private, and shared+private) without needing something like a tri-sta=
te
> > > enum.
> > >
> > > Link: https://lore.kernel.org/all/ZJX0hk+KpQP0KUyB@google.com
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
> > >  include/linux/kvm_host.h | 2 ++
> > >  virt/kvm/kvm_main.c      | 7 +++++++
> > >  2 files changed, 9 insertions(+)
> > >
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index 96aa930536b1..89c1a991a3b8 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -263,6 +263,8 @@ struct kvm_gfn_range {
> > >         gfn_t start;
> > >         gfn_t end;
> > >         union kvm_mmu_notifier_arg arg;
> > > +       bool only_private;
> > > +       bool only_shared;
> >
> > If these flags aren't used in this patch series, should this patch be
> > moved to the other series?
>
> If *both* TDX and SNP need this patch, then I think it's probably worth a=
pplying
> it now to make their lives easier.  But if only one needs the support, th=
en I
> completely agree this should be punted to whichever series needs it (this=
 also
> came up in v11, but we didn't force the issue).
>
> Mike, Isaku?
>
> > Also, if shared+private is a possibility, doesn't the prefix "only_"
> > confuse things a bit? I.e., what is shared+private, is it when both
> > are 0 or when both are 1? I assume it's the former (both are 0), but
> > it might be clearer.
>
> Heh, I was hoping that "only_private && only_shared" would be obviously n=
onsensical.
>
> The only alternative I can think would be to add an enum, e.g.
>
>         enum {
>                 PROCESS_PRIVATE_AND_SHARED,
>                 PROCESS_ONLY_PRIVATE,
>                 PROCESS_ONLY_SHARED,
>         };
>
> because every other way of expressing the flags either results in more co=
nfusion
> or an unsafe default.  I.e. I want zapping only private or only shared to=
 require
> the caller to explicitly set a non-zero value, which is how I ended up wi=
th
> "only_{private,shared}" as opposed to "process_{private,shared}".

I don't have a strong opinion about this. Having an enum looks good to me.

Cheers,
/fuad