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

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Message-ID: <aPemUXkliqL7QShY@char.us.oracle.com>
Date: Tue, 21 Oct 2025 11:27:13 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To: Jon Kohler <jon@...anix.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 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://blogs.oracle.com/linux/post/qemu-live-update

That should solve your QEMU problem.