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

[John Levon]

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

 - change the 0x1f handling to always report topology i.e. never report all
   zeroes

Yes, that would mean that if something requests 0x1f leaf even though the max
leaf is lower, they'd get data back, but it's not clear why that'd be an issue?

regards
john