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Message-ID: <CA+EHjTwXdntKGMjb8Zj1M3iVVN5NX6iLwbzJjE72j_eVczTQYw@mail.gmail.com>
Date: Thu, 2 Nov 2023 14:16:49 +0000
From: Fuad Tabba <tabba@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
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Subject: Re: [PATCH v13 18/35] KVM: x86: "Reset" vcpu->run->exit_reason early
in KVM_RUN
On Fri, Oct 27, 2023 at 7:23 PM Sean Christopherson <seanjc@...gle.com> 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@...gle.com>
> ---
Reviewed-by: Fuad Tabba <tabba@...gle.com>
Tested-by: Fuad Tabba <tabba@...gle.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 = KVM_EXIT_UNKNOWN;
> vcpu->arch.l1tf_flush_l1d = true;
>
> for (;;) {
> --
> 2.42.0.820.g83a721a137-goog
>
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