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Re: [PATCH v2 1/9] x86/cpuid: Advertise SSB_NO to guests by default
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Mon, 31 Jan 2022 11:15:09 +0000
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>
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- Thread-topic: [PATCH v2 1/9] x86/cpuid: Advertise SSB_NO to guests by default
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.
~Andrew
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