Re: [PATCH v2 1/3] amd/msr: implement VIRT_SPEC_CTRL for HVM guests on top of SPEC_CTRL

[Jan Beulich <jbeulich@suse.com>]

On 15.03.2022 15:18, Roger Pau Monne wrote:
> Use the logic to set shadow SPEC_CTRL values in order to implement
> support for VIRT_SPEC_CTRL (signaled by VIRT_SSBD CPUID flag) for HVM
> guests. This includes using the spec_ctrl vCPU MSR variable to store
> the guest set value of VIRT_SPEC_CTRL.SSBD, which will be OR'ed with
> any SPEC_CTRL values being set by the guest.
> 
> On hardware having SPEC_CTRL VIRT_SPEC_CTRL will not be offered by
> default to guests. VIRT_SPEC_CTRL will only be part of the max CPUID
> policy so it can be enabled for compatibility purposes.
> 
> Note that '!' is used in order to tag the VIRT_SSBD feature as
> specially handled. It's possible for the feature to be available to
> guests on hardware that doesn't support it natively, for example when
> implemented as done by this patch on top of SPEC_CTRL.SSBD (AMD_SSBD).

Except for this ! aspect the change looks good to me, but in order to
give my R-b this aspect needs sorting. Andrew - what are your thoughts
here? The reason cited by Roger doesn't look to be one that I so far
understood would require use of !, but your intentions may well have
been different from what my current understanding is.

Jan