Re: [edk2-devel] [PATCH 1/2] UefiCpuPkg/MpInitLib: Use AsmCpuidEx() for CPUID_EXTENDED_TOPOLOGY leaf

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

On 11/6/23 17:15, Tom Lendacky wrote:
> On 11/6/23 16:45, Lendacky, Thomas via groups.io wrote:
>> The CPUID_EXTENDED_TOPOLOGY CPUID leaf takes a subleaf as input when
>> returning CPUID information. However, the AsmCpuid() function does not
>> zero out ECX before the CPUID instruction, so the input leaf is used as
>> the sub-leaf for the CPUID request and returns erroneous/invalid CPUID
>> data, since the intent of the request was to get data related to sub-leaf
>> 0. Instead, use AsmCpuidEx() for the CPUID_EXTENDED_TOPOLOGY leaf.
> 
> Alternatively, the AsmCpuid() function could be changed to XOR ECX before 
> invoking the CPUID instruction. This would ensure that the 0 sub-leaf is 
> returned for any CPUID leaves that support sub-leaves. Thoughts?
> 
> Adding some additional maintainers for their thoughts, too.

Any thoughts on this approach (as a separate, unrelated patch) to 
eliminate future issues that could pop up?

Seems like zeroing out ECX before calling CPUID would be an appropriate 
thing to do, but I'm not sure if that will have any impact on the existing 
code base... it shouldn't, but you never know.

Thanks,
Tom

> 
> Thanks,
> Tom
> 
>>
>> Fixes: d4d7c9ad5fe5 ("UefiCpuPkg/MpInitLib: use BSP to do extended ...")
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>   UefiCpuPkg/Library/MpInitLib/AmdSev.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/UefiCpuPkg/Library/MpInitLib/AmdSev.c 
>> b/UefiCpuPkg/Library/MpInitLib/AmdSev.c
>> index bda4960f6fd3..d34f9513e002 100644
>> --- a/UefiCpuPkg/Library/MpInitLib/AmdSev.c
>> +++ b/UefiCpuPkg/Library/MpInitLib/AmdSev.c
>> @@ -256,7 +256,14 @@ FillExchangeInfoDataSevEs (
>>     if (StdRangeMax >= CPUID_EXTENDED_TOPOLOGY) {
>>       CPUID_EXTENDED_TOPOLOGY_EBX  ExtTopoEbx;
>> -    AsmCpuid (CPUID_EXTENDED_TOPOLOGY, NULL, &ExtTopoEbx.Uint32, NULL, 
>> NULL);
>> +    AsmCpuidEx (
>> +      CPUID_EXTENDED_TOPOLOGY,
>> +      0,
>> +      NULL,
>> +      &ExtTopoEbx.Uint32,
>> +      NULL,
>> +      NULL
>> +      );
>>       ExchangeInfo->ExtTopoAvail = !!ExtTopoEbx.Bits.LogicalProcessors;
>>     }
>>   }


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