Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
MIME-Version: 1.0
References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-19-seanjc@google.com>
In-Reply-To: <20231027182217.3615211-19-seanjc@google.com>
From:   Fuad Tabba <tabba@google.com>
Date:   Thu, 2 Nov 2023 14:16:49 +0000
Message-ID: <CA+EHjTwXdntKGMjb8Zj1M3iVVN5NX6iLwbzJjE72j_eVczTQYw@mail.gmail.com>
Subject: Re: [PATCH v13 18/35] KVM: x86: "Reset" vcpu->run->exit_reason early
 in KVM_RUN
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>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Fri, Oct 27, 2023 at 7:23=E2=80=AFPM Sean Christopherson <seanjc@google.=
com> wrote:
>
> Initialize run->exit_reason to KVM_EXIT_UNKNOWN early in KVM_RUN to reduc=
e
> the probability of exiting to userspace with a stale run->exit_reason tha=
t
> *appears* to be valid.
>
> To support fd-based guest memory (guest memory without a corresponding
> userspace virtual address), KVM will exit to userspace for various memory
> related errors, which userspace *may* be able to resolve, instead of usin=
g
> e.g. BUS_MCEERR_AR.  And in the more distant future, KVM will also likely
> utilize the same functionality to let userspace "intercept" and handle
> memory faults when the userspace mapping is missing, i.e. when fast gup()
> fails.
>
> Because many of KVM's internal APIs related to guest memory use '0' to
> indicate "success, continue on" and not "exit to userspace", reporting
> memory faults/errors to userspace will set run->exit_reason and
> corresponding fields in the run structure fields in conjunction with a
> a non-zero, negative return code, e.g. -EFAULT or -EHWPOISON.  And becaus=
e
> KVM already returns  -EFAULT in many paths, there's a relatively high
> probability that KVM could return -EFAULT without setting run->exit_reaso=
n,
> in which case reporting KVM_EXIT_UNKNOWN is much better than reporting
> whatever exit reason happened to be in the run structure.
>
> Note, KVM must wait until after run->immediate_exit is serviced to
> sanitize run->exit_reason as KVM's ABI is that run->exit_reason is
> preserved across KVM_RUN when run->immediate_exit is true.
>
> Link: https://lore.kernel.org/all/20230908222905.1321305-1-amoorthy@googl=
e.com
> Link: https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>

Cheers,
/fuad

>  arch/x86/kvm/x86.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ee3cd8c3c0ef..f41dbb1465a0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10963,6 +10963,7 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
>  {
>         int r;
>
> +       vcpu->run->exit_reason =3D KVM_EXIT_UNKNOWN;
>         vcpu->arch.l1tf_flush_l1d =3D true;
>
>         for (;;) {
> --
> 2.42.0.820.g83a721a137-goog
>