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Re: [Xen-devel] [PATCH] libx86: Elide more empty CPUID leaves when serialising a policy


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 23 May 2019 11:38:27 +0100
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  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 23 May 2019 10:38:42 +0000
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On 23/05/2019 09:33, Jan Beulich wrote:
>>>> On 22.05.19 at 17:50, <andrew.cooper3@xxxxxxxxxx> wrote:
>> x86_cpuid_copy_to_buffer() currently serialises the full content of the
>> various subleaf unions.  While leaves 4, 0xb and 0xd don't have a concrete
>> max_subleaf field, they do have well defined upper bounds.
>>
>> Diffing the results of `xen-cpuid -p` shows the resutling saving:
>>
>>   @@ -1,5 +1,5 @@
>>    Xen reports there are maximum 114 leaves and 1 MSRs
>>   -Raw policy: 93 leaves, 1 MSRs
>>   +Raw policy: 38 leaves, 1 MSRs
>>     CPUID:
>>      leaf     subleaf  -> eax      ebx      ecx      edx
>>      00000000:ffffffff -> 00000016:756e6547:6c65746e:49656e69
>>   @@ -32,7 +32,7 @@ Raw policy: 93 leaves, 1 MSRs
>>     MSRs:
>>      index    -> value
>>      000000ce -> 0000000080000000
>>   -Host policy: 93 leaves, 1 MSRs
>>   +Host policy: 33 leaves, 1 MSRs
>>     CPUID:
>>      leaf     subleaf  -> eax      ebx      ecx      edx
>>      00000000:ffffffff -> 0000000d:756e6547:6c65746e:49656e69
>>
>> which is mostly due to no longer writing out 64 leaves for xstate when (on
>> this CoffeeLake system) 8 will do.
>>
>> Extend the unit tests to cover empty and partially filled subleaf unions.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> For the lib/x86/ part
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

> For the test harness part
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
> No idea how else I should represent that I didn't look overly closely
> at the harness additions.

Well - I can state that the additions to the test harness did find bugs.

Overall, I think the content of tools/tests/ is of relatively little
importance in the grand scheme of things.  I certainly don't spend as
much time reviewing the test_x86_emulator changes as the changes to
x86_emulate() itself.

~Andrew

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