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Re: [PATCH] x86/ucode: Fix stability of the Raw CPU Policy rescan


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 24 Jan 2024 17:23:00 +0000
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  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Wed, 24 Jan 2024 17:23:10 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 24/01/2024 3:37 pm, Jan Beulich wrote:
> On 23.01.2024 21:59, Andrew Cooper wrote:
>> Always run microcode_update_helper() on the BSP, so the the updated Raw CPU
>> policy doesn't get non-BSP topology details included.
> Wouldn't it be better (and consistent with ...
>
>> Have calculate_raw_cpu_policy() clear the instantanious XSTATE sizes.  The
>> value XCR0 | MSR_XSS had when we scanned the policy isn't terribly 
>> interesting
>> to report.
> ... this) to purge these details from the raw policy as well then?

I did spend some time considering this.

Rerunning on the same CPU is more resilient to new topology leaves, so
we'd want to be doing that irrespective.

The XCR0/XSS state really is transient, and the useful information is
everywhere else in leaf 0xd.

>
>> When CPUID Masking is active, it affects CPUID instructions issued by Xen
>> too.  Transiently disable masking to get a clean scan.
>>
>> Fixes: 694d79ed5aac ("x86/ucode: Refresh raw CPU policy after microcode 
>> load")
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Irrespective of the question above, I'm also okay with the change as is:
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

~Andrew



 


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