linux-kernel - Re: [PATCH v2 02/21] KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS

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Message-ID: <CALMp9eRm_n4s-ryw5-JiD8-5S+r+F15pSFsQSqava6gHrYjJBg@mail.gmail.com>
Date:   Wed, 6 Jul 2022 13:02:52 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH v2 02/21] KVM: VMX: Drop bits 31:16 when shoving exception
 error code into VMCS

On Tue, Jun 14, 2022 at 1:47 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Deliberately truncate the exception error code when shoving it into the
> VMCS (VM-Entry field for vmcs01 and vmcs02, VM-Exit field for vmcs12).
> Intel CPUs are incapable of handling 32-bit error codes and will never

Never say "never." :-)

> generate an error code with bits 31:16, but userspace can provide an
> arbitrary error code via KVM_SET_VCPU_EVENTS.  Failure to drop the bits
> on exception injection results in failed VM-Entry, as VMX disallows
> setting bits 31:16.  Setting the bits on VM-Exit would at best confuse
> L1, and at worse induce a nested VM-Entry failure, e.g. if L1 decided to
> reinject the exception back into L2.
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>