Re: [PATCH v2 4/4] target/i386: Update CMPLegacy handling for Zhaoxin CPUs

[Zhao Liu]

On Thu, Aug 08, 2024 at 09:44:18PM -0400, Ewan Hai wrote:
> Date: Thu, 8 Aug 2024 21:44:18 -0400
> From: Ewan Hai <ewanhai-oc@zhaoxin.com>
> Subject: Re: [PATCH v2 4/4] target/i386: Update CMPLegacy handling for
>  Zhaoxin CPUs
> 
> 
> Hi Zhao Liu,
> 
> Thank you for your feedback.
> 
> On 8/8/24 06:30, Zhao Liu wrote:
> > Hi EwanHai,
> > 
> > On Thu, Jul 04, 2024 at 07:25:11AM -0400, EwanHai wrote:
> > > Date: Thu, 4 Jul 2024 07:25:11 -0400
> > > From: EwanHai <ewanhai-oc@zhaoxin.com>
> > > Subject: [PATCH v2 4/4] target/i386: Update CMPLegacy handling for Zhaoxin
> > >   CPUs
> > > X-Mailer: git-send-email 2.34.1
> > > 
> > > Zhaoxin CPUs handle the CMPLegacy bit in the same way
> > > as Intel CPUs.

Here it could be clearer to say "Don't set up CMPLegacy bit in
CPUID[0x80000001].ecx for VIA/Zhaoxin CPUs".

> This patch simplifies the existing logic by
> > > using the IS_XXX_CPU macro and includes checks for Zhaoxin
> > > vendor to align their behavior with Intel.
> > > 
> > > Signed-off-by: EwanHai <ewanhai-oc@zhaoxin.com>
> > > ---
> > >   target/i386/cpu.c | 4 +---
> > >   1 file changed, 1 insertion(+), 3 deletions(-)
> > > 
> > > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > > index a3747fc487..c52a4cf3ba 100644
> > > --- a/target/i386/cpu.c
> > > +++ b/target/i386/cpu.c
> > > @@ -6945,9 +6945,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t 
> > > index, uint32_t count,
> > >            * So don't set it here for Intel to make Linux guests happy.
> > >            */
> > >           if (threads_per_pkg > 1) {
> > > -            if (env->cpuid_vendor1 != CPUID_VENDOR_INTEL_1 ||
> > > -                env->cpuid_vendor2 != CPUID_VENDOR_INTEL_2 ||
> > > -                env->cpuid_vendor3 != CPUID_VENDOR_INTEL_3) {
> > > +            if (!IS_INTEL_CPU(env) && !IS_ZHAOXIN_CPU(env)) {
> > This change implicitly changes the behavior of existing VIA CPU.
> > 
> > Is this a bug for the original VIA? If so, I suggest a separate patch to
> > fix it and explain the effect on the VIA (Zhaoxin1) CPU.
> > 
> > Regards,
> > Zhao
>
> The reason for this change is not due to a discovered bug, but rather
> because both Centaurhauls and Shanghai CPUs follow Intel’s behavior
> regarding the CMPLegacy bit. Specifically, AMD CPUs enumerate the
> threads per package information in the CPUID leaf 0x80000001 output
> ECX register, while Intel (and **other processors following Intel’s
> behavior**) do not. Therefore, this modification is simply intended to
> logically supplement the existing code.

I see, thanks.

> Given this, do you think it would be appropriate for me to submit
> a separate patch to explain this behavior and its effect on
> VIA (Zhaoxin1) CPUs? If so, I will submmit this change in a separate
> patch.

I think there's no need to split this.

However, I think it's necessary to state the effect of the change in
the changelog/commit message. It's also worth stating if it won't have
any effect on the OS/software. Afterall, the comment of this bit said
it affects Linux kernel.

Also, changes to the old VIA behavior are worth stating in the commit
message, i.e., this patch's changes to Zhaoxin CPUs include the previous
VIA CPUs.

Additionally, considering this change is to fix the CPUID which doesn't
match the bare metal, then what about changing the subject to

"target/i386: Mask CMPLegacy bit in CPUID[0x80000001].ecx for Zhaoxin/VIA
CPUs"?

Thanks,
Zhao

> > >                   *ecx |= 1 << 1;    /* CmpLegacy bit */
> > >               }
> > >           }
> > > --
> > > 2.34.1
> > > 
>