Re: [PATCH v3 13/15] x86/cpu/intel: Bound the non-architectural constant_tsc model checks

[Xiaoyao Li <xiaoyao.li@xxxxxxxxx>]

On 8/22/2025 3:43 AM, Sohil Mehta wrote:
> On 8/21/2025 12:34 PM, Sohil Mehta wrote:
> > On 8/21/2025 6:15 AM, David Woodhouse wrote:
> >
> > > Hm. My test host is INTEL_HASWELL_X (0x63f). For reasons which are
> > > unclear to me, QEMU doesn't set bit 8 of 0x80000007 EDX unless I
> > > explicitly append ',+invtsc' to the existing '-cpu host' on its command
> > > line. So now my guest doesn't think it has X86_FEATURE_CONSTANT_TSC.
> > Haswell should have X86_FEATURE_CONSTANT_TSC, so I would have expected
> > the guest bit to be set. Until now, X86_FEATURE_CONSTANT_TSC was set
> > based on the Family-model instead of the CPUID enumeration which may
> > have hid the issue.
> Correction:
> s/instead/as well as
>
> >  From my initial look at the QEMU implementation, this seems intentional.
> >
> > QEMU considers Invariant TSC as un-migratable which prevents it from
> > being exposed to migratable guests (default).
> > target/i386/cpu.c:
> > [FEAT_8000_0007_EDX]
> >           .unmigratable_flags = CPUID_APM_INVTSC,
> >
> > Can you please try '-cpu host,migratable=off'?
> This is mainly to verify. If confirmed, I am not sure what the long term
> solution should be.
yeah. It's the intentional behavior of QEMU.

Invariant TSC is ummigratable unless users explicitly configures the TSC 
frequency, e.g., "-cpu host,tsc-frequency=xxx". Because the TSC 
frequency is by default the host's frequency if no "tsc-frequency" 
specified, and it will change when the VM is migrated to a host with a 
different TSC frequency.
It's the specific behavior/rule of QEMU. We just need to keep it in 
mind. If we want to expose invariant TSC to the guest with QEMU's "-cpu 
host", we can either:
1) explicitly configure the "tsc-frequency", or
2) explicitly turn off "migratable"