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Re: [PATCH v2 1/9] x86/cpuid: Advertise SSB_NO to guests by default
- To: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Mon, 31 Jan 2022 12:23:13 +0100
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Mon, Jan 31, 2022 at 11:15:09AM +0000, Andrew Cooper wrote:
> On 31/01/2022 09:41, Roger Pau Monné wrote:
> > On Fri, Jan 28, 2022 at 01:29:19PM +0000, Andrew Cooper wrote:
> >> This is a statement of hardware behaviour, and not related to controls for
> >> the
> >> guest kernel to use. Pass it straight through from hardware.
> >>
> > Not really related to this patch per se, but I think we should expose
> > AMD_SSBD unconditionally for SPEC_CTRL (and VIRT_SSBD for
> > VIRT_SPEC_CTRL when supported) in the max policies and implement them
> > as noop for compatibility reasons?
> >
> > I would expect CPUs exposing SSB_NO to drop AMD_SSBD and VIRT_SSBD at
> > some point.
>
> Why? SSBD is an architectural feature. It's far more likely to turn
> into a no-op on SSB_NO hardware, than to disappear, especially as the
> MSR is exposed to guests.
>
> VIRT_SSBD is only offered by hypervisors, and should only be offered
> when there are members in the migration pool without MSR_SPEC_CTRL.
But we should also offer VIRT_SSBD in the max policy if the hardware
reports SSB_NO, because then it's a no-op and would allow for
migration from boxes where we do offer VIRT_SSBD.
Thanks, Roger.
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