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SubjectRe: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD down to the guest
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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.

Paolo

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