Re: [PATCH v6] i386/cpu: fixup number of addressable IDs for logical processors in the physical package

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 12/3/2024 3:33 PM, Zhao Liu wrote:
>> However, back to the patch, I think we cannot change it as this patch
>> directly. Instead, we need a compat_props for the changed behavior, because
>> this isn't a bug fix and it introduces guest-visible differences.
> 
> This is a fix, not a new feature, so compat_props is not needed.

Fix what? QEMU behaves as it for so many years and if the guest OS uses 
the algorithm recommended by SDM, there is no issue.

>> For ancient Intel CPUs, EBX[23:16] did represent the number of Logical
>> processor per package. I believe this should be the reason why QEMU
>> implemented it as is:
>>
>>    - on SDM version 013, EBX[23:16]: Number of logical processors per
>> physical processor; two for the Pentium 4 processor supporting
>> Hyper-Threading Technology.
>>
>>    - on SDM version 015, it changed to: Number of initial APIC IDs reserved
>> for this physical package. Normally, this is the number of logical
>> processors per physical package.
>>
>>    - on SDM version 016, it changed to: Maximum number of logical processors
>> in this physical package.
>>
>>    - finally, starting from SDM version 026, it changed to what reads now:
>> Maximum number of addressable IDs for logical processors in this physical
>> package.
> 
> And this is an architecturally defined CPUID, so SDM ensures backward
> compatibility.

SDM ensure the backwards compatibility by recommending to round the 
number up to the power-of 2 when using it to calculate the topology with 
legacy method.

> Regards,
> Zhao
>