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From:   Fuad Tabba <tabba@google.com>
Date:   Thu, 2 Nov 2023 14:01:39 +0000
Message-ID: <CA+EHjTyAU9XZ3OgqXjmAKh-BKsLrH_8QtnJihQxF4fhk8WPSYg@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>,
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        Andrew Morton <akpm@linux-foundation.org>, kvm@vger.kernel.org,
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Hi,

On Fri, Oct 27, 2023 at 7:22=E2=80=AFPM Sean Christopherson <seanjc@google.=
com> wrote:
>
> Add flags to "struct kvm_gfn_range" to let notifier events target only
> shared and only private mappings, and write up the existing mmu_notifier
> 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-state
> 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?

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.

Cheers,
/fuad

>         bool may_block;
>  };
>  bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index cb9376833c18..302ccb87b4c1 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -635,6 +635,13 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva=
_range(struct kvm *kvm,
>                          * the second or later invocation of the handler)=
.
>                          */
>                         gfn_range.arg =3D range->arg;
> +
> +                       /*
> +                        * HVA-based notifications aren't relevant to pri=
vate
> +                        * mappings as they don't have a userspace mappin=
g.
> +                        */
> +                       gfn_range.only_private =3D false;
> +                       gfn_range.only_shared =3D true;
>                         gfn_range.may_block =3D range->may_block;
>
>                         /*
> --
> 2.42.0.820.g83a721a137-goog
>