Re: [PATCH v4 04/19] target/i386/cpu: Remove X86CPU::check_cpuid field

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 28 May 2025 13:23:49 +0800
Zhao Liu <zhao1.liu@intel.com> wrote:

> On Wed, May 28, 2025 at 10:09:56AM +0800, Xiaoyao Li wrote:
> > Date: Wed, 28 May 2025 10:09:56 +0800
> > From: Xiaoyao Li <xiaoyao.li@intel.com>
> > Subject: Re: [PATCH v4 04/19] target/i386/cpu: Remove X86CPU::check_cpuid
> >  field
> > 
> > On 5/12/2025 4:39 PM, Philippe Mathieu-Daudé wrote:  
> > > The X86CPU::check_cpuid boolean was only set in the
> > > pc_compat_2_4[] array, via the 'check=off' property.
> > > We removed all machines using that array, lets remove
> > > that CPU property and simplify x86_cpu_realizefn().  
> > 
> > No.
> > 
> > We cannot do this. Because it changes the behavior of QEMU.
> > 
> > 'check_cpuid' is true by default while 'enforce_cpuid' is false. So that
> > QEMU emits warnings in x86_cpu_filter_features() by default when user
> > requests unsupported CPU features. If remove "check" property and the
> > internal 'check_cpuid', QEMU will not do it unless user sets enforce_cpuid
> > explicitly.  
> 
> One option would be to have x86_cpu_filter_features() unconditionally
> turn on verbose and print warnings, but some people might want to turn
> off these warning prints, I don't know if anyone would, but it would be
> possible.
> 
> The other option is still to keep the “check” property.
> 
> IMO, the latter option is the better way to reduce Philippe's burden.

we essentially loose warnings by default when some features aren't available,
qemu still continues to run though.

Given that Daniel acked it from libvirt side, libvirt doesn't care about warnings
(it does its has its own cpu model calculation). Likely other mgmt do not care
about it either, and if they do they probably doing something wrong and
should use QMP to get that data.
That leaves us with human users, for that case I'd say one should use
enforce_cpuid if feature availability matters.

so +1 to removal
 
> 
> Regards,
> Zhao
> 
>