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

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Message-ID: <5E09F6D2-F6E5-45A2-8264-34DC6DF679B5@nutanix.com>
Date: Tue, 21 Oct 2025 15:35:28 +0000
From: Jon Kohler <jon@...anix.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC: Dave Hansen <dave.hansen@...el.com>,
        Pawan Gupta
	<pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh
 Poimboeuf <jpoimboe@...nel.org>,
        Jonathan Corbet <corbet@....net>, Ingo
 Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Brian
 Gerst <brgerst@...il.com>,
        Brendan Jackman <jackmanb@...gle.com>,
        "Ahmed S.
 Darwish" <darwi@...utronix.de>,
        Alexandre Chartre
	<alexandre.chartre@...cle.com>,
        "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out



> On Oct 21, 2025, at 11:27 AM, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> 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@...el.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)?