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

Jim Mattson
2019-06-20T18:41:36Z

On Thu, Jun 20, 2019 at 4:02 AM Vitaly Kuznetsov <vkuznets@redhat.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@google.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>