linux-kernel - Re: [PATCH RFC 5/5] x86: KVM: svm: remove hardcoded instruction length from intercepts

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Date:   Thu, 20 Jun 2019 11:41:22 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH RFC 5/5] x86: KVM: svm: remove hardcoded instruction
 length from intercepts

On Thu, Jun 20, 2019 at 4:02 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Various intercepts hard-code the respective instruction lengths to optimize
> skip_emulated_instruction(): when next_rip is pre-set we skip
> kvm_emulate_instruction(vcpu, EMULTYPE_SKIP). The optimization is, however,
> incorrect: different (redundant) prefixes could be used to enlarge the
> instruction. We can't really avoid decoding.
>
> svm->next_rip is not used when CPU supports 'nrips' (X86_FEATURE_NRIPS)
> feature: next RIP is provided in VMCB. The feature is not really new
> (Opteron G3s had it already) and the change should have zero affect.
>
> Remove manual svm->next_rip setting with hard-coded instruction lengths. The
> only case where we now use svm->next_rip is EXIT_IOIO: the instruction
> length is provided to us by hardware.
>
> Reported-by: Jim Mattson <jmattson@...gle.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>