Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out

[Jon Kohler <jon@nutanix.com>]

> On Oct 21, 2025, at 11:27 AM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> 
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> On Tue, Oct 21, 2025 at 02:39:15PM +0000, Jon Kohler wrote:
>> 
>> 
>>> On Oct 21, 2025, at 10:01 AM, Dave Hansen <dave.hansen@intel.com> wrote:
>>> 
>>>>> 
>>> On 10/21/25 06:40, Jon Kohler wrote:
>>>> So to simplify it down:
>>>> A guest VM that updates to a ITS-enabled guest kernel sees performance
>>>> impacts on non-vulnerable hardware, when running on non-BHI_CTRL and/or
>>>> non-ITS_NO hypervisors, which is a very easy situation to get into, especially
>>>> on QEMU with live migration-enabled pools.
>>> 
>>> By non-$FEATURE, do you mean that they chose to not enumerate those
>>> features, or that they are completely ignorant of them?
>> 
>> Both cases are true for QEMU.
>> 
>> For ITS_NO, it is an allowed feature, but its not part of a QEMU model by
>> default, so the higher level control plane whatever that may be would need to
>> specifically turn it on, its not automatic.
>> 
>> For BHI_CTRL, depending on what QEMU the VM was originally *started* on,
>> the guest may have access to Sapphire Rapids models, but BHI_CTRL may
>> not have existed in the QEMU source at that time, as those were introduced
>> into two different timeframes.
> 
> QEMU provides now a mechanism to update itself to a newer version. See
> 
> https://urldefense.proofpoint.com/v2/url?u=https-3A__blogs.oracle.com_linux_post_qemu-2Dlive-2Dupdate&d=DwIFaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=UUSvpPViiTB6CJQtj3GREK4bQFz7MT9uNiAu5AL3O23d6I1yk4vheLDyR41ZcbRI&s=x19gwHU3HGXSlGkK0ppkawex3SIbs8xHj5hPtwNCFwc&e= 
> 
> That should solve your QEMU problem.

Can this live update feature change CPU feature bits (e.g. add on
-cpu ModelHere … new_flag=yes) during the update?

Can it change CPU models during the live update (e.g. change
-cpu Model-v1 to -cpu Model-v2)?