Re: [Patch 2/6] KVM: VMX: Use wrmsr for switching between guest and host IA32_XSS

Vitaly Kuznetsov
2019-10-09T12:34:24Z

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 09/10/19 02:41, Aaron Lewis wrote:
>> Set IA32_XSS for the guest and host during VM Enter and VM Exit
>> transitions rather than by using the MSR-load areas.
>>
>> Reviewed-by: Jim Mattson <jmattson@google.com>
>> Signed-off-by: Aaron Lewis <aaronlewis@google.com>
>
> This commit message is missing an explanation of why this is a good thing.
>
> Also, the series is missing a cover letter that explains a bit more of
> the overall picture. I have no problem with no cover letter for
> two-patch series, but at six it is definitely a requirement.
>
> So I'm replying to this patch as a proxy for the whole series, and
> asking: why is it useful to enable XSAVES (on AMD or anywhere) if anyway
> IA32_XSS is limited to zero?

I know at least one good reason to do so: Hyper-V 2016 Gen1 (but not
Gen2!) doesn't seem to be able to boot without XSAVES (don't ask me
why). I'm not particularly sure if Aaron is aiming at fixing this one
though.

--
Vitaly