Messages in this thread | | | Subject | Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest | From | Nadav Amit <> | Date | Tue, 9 Jan 2018 21:03:27 -0800 |
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Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 09/01/2018 17:48, Liran Alon wrote: >>>> + if (have_spec_ctrl) { >>>> + rdmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl); >>>> + if (vmx->spec_ctrl != 0) >>>> + wrmsrl(MSR_IA32_SPEC_CTRL, 0); >> >> As I said also on the AMD patch, I think this is a bug. >> Intel specify that we should set IBRS bit even if it was already set on every #VMExit. > > That's correct (though I'd like to understand _why_---I'm not inclined > to blindly trust a spec), but for now it's saving a wrmsr of 0. That is > quite obviously okay, and will be also okay after the bare-metal IBRS > patches. > > Of course the code will become something like > > if (using_ibrs || vmx->spec_ctrl != 0) > wrmsrl(MSR_IA32_SPEC_CTRL, host_ibrs); > > optimizing the case where the host is using retpolines.
Excuse my ignorance: Can you point me to the specifications that mention “we should set IBRS bit even if it was already set on every #VMExit” ?
Thanks, Nadav
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