Re: [PATCH v3 7/9] x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.02.2022 13:28, Andrew Cooper wrote:
> On 01/02/2022 11:47, Jan Beulich wrote:
>> On 31.01.2022 16:36, Andrew Cooper wrote:
>>> Hardware maintains both host and guest versions of MSR_SPEC_CTRL, but guests
>>> run with the logical OR of both values.  Therefore, in principle we want to
>>> clear Xen's value before entering the guest.  However, for migration
>>> compatibility,
>> I think you've explained this to me before, but I can't seem to put
>> all of it together already now. Could expand on how a non-zero value
>> behind a guest's back can help with migration compatibility? At the
>> first glance I would be inclined to say only what the guest actually
>> gets to see and use can affect its migration.
> 
> For VMs which see VIRT_SPEC_CTRL (compatibility with Fam15 thru Zen1),
> writes of VIRT_SPEC_CTRL.SSBD (probably) need to use
> SSBD-behind-the-guest's back.  I say probably, because I think this is
> the least bad implementation option, but until we have working support,
> it's still a guess.

So this is future work (and mentioning just this in the description
would be enough to address my comment), but ...

> For the ultra paranoid, a VM migrating in which can't see PSFD (e.g. for
> compatibility with Zen2) should have PSFD set behind it's back.

... this is something we should be doing right away then?

Jan