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

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

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"