Re: [RFC 06/10] i386/cpu: Introduce enable_cpuid_0x1f to force exposing CPUID 0x1f

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

On 5/14/2025 11:23 PM, Zhao Liu wrote:
> Hi Igor, thanks for your review!
> 
> On Tue, May 13, 2025 at 02:45:15PM +0200, Igor Mammedov wrote:
>> Date: Tue, 13 May 2025 14:45:15 +0200
>> From: Igor Mammedov <imammedo@redhat.com>
>> Subject: Re: [RFC 06/10] i386/cpu: Introduce enable_cpuid_0x1f to force
>>   exposing CPUID 0x1f
>> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.43; x86_64-redhat-linux-gnu)
>>
>> On Wed, 23 Apr 2025 19:46:58 +0800
>> Zhao Liu <zhao1.liu@intel.com> wrote:
>>
>>> From: Xiaoyao Li <xiaoyao.li@intel.com>
>>>
>>> Currently, QEMU exposes CPUID 0x1f to guest only when necessary, i.e.,
>>> when topology level that cannot be enumerated by leaf 0xB, e.g., die or
>>> module level, are configured for the guest, e.g., -smp xx,dies=2.
>>>
>>> However, TDX architecture forces to require CPUID 0x1f to configure CPU
>>> topology.
>>>
>>> Introduce a bool flag, enable_cpuid_0x1f, in CPU for the case that
>>> requires CPUID leaf 0x1f to be exposed to guest.
>>>
>>> Introduce a new function x86_has_cpuid_0x1f(), which is the warpper of
>>> cpu->enable_cpuid_0x1f and x86_has_extended_topo() to check if it needs
>>> to enable cpuid leaf 0x1f for the guest.
>>
>> that reminds me about recent attempt to remove enable_cpuid_0xb,
>>
>> So is enable_cpuid_0x1f inteded to be used by external users or
>> it's internal only knob for TDX sake?
> 
> TDX needs this and I also try to apply this to named CPU models. For
> max/host CPUs, there are no explicit use cases. I think it's enough to
> make named CPU models have 0x1f.
> 
> Then this should be only used internally.
> 
>> I'd push for it being marked as internal|unstable with the means
>> we currently have (i.e. adding 'x-' prefix)
> 
> Sure, 'x-' is good. (If there is the internal property in the future,
> I can also convert this unsatble property into internal one.)
> 
> This patch is picked from the TDX series, and in this patch Xiaoyao
> doesn't make 0x1f enabling as property. In the next patch (RFC patch 7),
> I add the "cpuid-0x1f" property. I'll rename that property as
> "x-cpuid-0x1f".
> 
>> and also enable_ is not right here, the leaf is enabled when
>> topology requires it.
>> perhaps s/enable_/force_/
> 
> Thanks, I agree force_cpuid_0x1f is a better name.
> 
> @Xiaoyao, do you agree with this idea?
> 
> But probably TDX won't have v10 though, I can rename the field in my v2
> after TDX.

I'm OK with it.

The TDX series won't be merged by Paolo soon. It has to be after the KVM 
part being in linux v6.16-rc1, i.e., about one month later.

And there are rebase conflicts when I rebase the TDX-QEMU series against 
upstream QEMU master daily. It seems to have a newer version of TDX-QEMU 
when it's going to be picked up by Paolo for Paolo's convenience. If a 
v10 needed, I can rename in it.

Let's see how it goes.

> Regards,
> Zhao
>