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Date:   Mon, 30 Oct 2023 18:31:50 +0100
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Subject: Re: [PATCH v13 18/35] KVM: x86: "Reset" vcpu->run->exit_reason early
 in KVM_RUN
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To:     Sean Christopherson <seanjc@google.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>
Cc:     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>,
        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?B?TWlja2HDq2wgU2FsYcO8?= =?UTF-8?Q?n?= <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>
References: <20231027182217.3615211-1-seanjc@google.com>
 <20231027182217.3615211-19-seanjc@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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In-Reply-To: <20231027182217.3615211-19-seanjc@google.com>
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On 10/27/23 20:22, Sean Christopherson wrote:
> Initialize run->exit_reason to KVM_EXIT_UNKNOWN early in KVM_RUN to reduce
> the probability of exiting to userspace with a stale run->exit_reason that
> *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 using
> 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 because
> KVM already returns  -EFAULT in many paths, there's a relatively high
> probability that KVM could return -EFAULT without setting run->exit_reason,
> 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@google.com
> Link: https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@google.com
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   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 = KVM_EXIT_UNKNOWN;
>   	vcpu->arch.l1tf_flush_l1d = true;
>   
>   	for (;;) {

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>