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

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Message-ID: <883EDC14-1A26-46F4-B9E9-C75A6DF07195@nutanix.com>
Date: Tue, 21 Oct 2025 14:39:15 +0000
From: Jon Kohler <jon@...anix.com>
To: Dave Hansen <dave.hansen@...el.com>
CC: 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 10:01 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> !-------------------------------------------------------------------|
>  CAUTION: External Email
> 
> |-------------------------------------------------------------------!
> 
> 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.