Re: [PATCH v1] target/i386: Always set leaf 0x1f

[Zhao Liu]

Hi Igor,

On Wed, Jul 24, 2024 at 02:54:32PM +0200, Igor Mammedov wrote:
> Date: Wed, 24 Jul 2024 14:54:32 +0200
> From: Igor Mammedov <imammedo@redhat.com>
> Subject: Re: [PATCH v1] target/i386: Always set leaf 0x1f
> X-Mailer: Claws Mail 4.3.0 (GTK 3.24.42; x86_64-redhat-linux-gnu)
> 
> On Wed, 24 Jul 2024 12:13:28 +0100
> John Levon <john.levon@nutanix.com> wrote:
> 
> > On Wed, Jul 24, 2024 at 03:59:29PM +0530, Manish wrote:
> > 
> > > > > Leaf 0x1f is superset of 0xb, so it makes sense to set 0x1f equivalent
> > > > > to 0xb by default and workaround windows issue.>
> > > > > This change adds a
> > > > > new property 'cpuid-0x1f-enforce' to set leaf 0x1f equivalent to 0xb 
> > > > > in
> > > > > case extended CPU topology is not configured and behave as before 
> > > > > otherwise.  
> > > > repeating question
> > > > why we need to use extra property instead of just adding 0x1f leaf for 
> > > > CPU models
> > > > that supposed to have it?  
> > > 
> > > As i mentioned in earlier response. "Windows expects it only when we have
> > > set max cpuid level greater than or equal to 0x1f. I mean if it is exposed
> > > it should not be all zeros. SapphireRapids CPU definition raised cpuid 
> > > level
> > > to 0x20, so we starting seeing it with SapphireRapids."
> > > 
> > > Windows does not expect 0x1f to be present for any CPU model. But if it is
> > > exposed to the guest, it expects non-zero values.  
> > 
> > I think Igor is suggesting:
> > 
> >  - leave x86_cpu_expand_features() alone completely
> yep, drop that if possible
> 
>  
> >  - change the 0x1f handling to always report topology i.e. never report all
> >    zeroes
> 
> Do this but only for CPU models that have this leaf per spec,
> to avoid live migration issues create a new version of CPU model,
> so it would apply only for new version. This way older versions
> and migration won't be affected. 

So that in the future every new Intel CPU model will need to always
enable 0x1f. Sounds like an endless game. So my question is: at what
point is it ok to consider defaulting to always enable 0x1f and just
disable it for the old CPU model?

Thanks,
Zhao