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 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.

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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