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

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 8/21/25 21:46, Xiaoyao Li wrote:
> 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"

Could the TSC frequency be included in the migration stream?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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