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Re: [PATCH v2 3/3] x86/amd: Use newer SSBD mechanisms if they exist
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 8 Sep 2021 12:19:38 +0100
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- Cc: Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 08 Sep 2021 11:19:50 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 08/09/2021 11:43, Jan Beulich wrote:
> On 07.09.2021 18:19, Andrew Cooper wrote:
>> The opencoded legacy Memory Disambiguation logic in init_amd() neglected
>> Fam19h for the Zen3 microarchitecture. In practice, all Zen2 based system
>> have the architectural MSR_SPEC_CTRL and the SSBD bit within it.
> Don't you mean Zen3 in the 2nd sentence? Otherwise there's a missing
> connect between both sentences.
No. Zen2/Rome has MSR_SPEC_CTRL.
The point is that Zen2 and later shouldn't be using MSR_AMD64_LS_CFG in
the first place. I'll tweak the wording.
>
>> Implement the algorithm given in AMD's SSBD whitepaper, and leave a
>> printk_once() behind in the case that no controls can be found.
>>
>> This now means that a user explicitly choosing `spec-ctrl=ssbd` will properly
>> turn off Memory Disambiguation on Fam19h/Zen3 systems.
>>
>> This still remains a single system-wide setting (for now), and is not context
>> switched between vCPUs. As such, it doesn't interact with Intel's use of
>> MSR_SPEC_CTRL and default_xen_spec_ctrl (yet).
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Thanks.
~Andrew
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