Paolo Bonzini <pbonzini@xxxxxxxxxx> 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
