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

[Zhao Liu]

> 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.

> 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.

Regards,
Zhao