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

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Message-ID: <20251020194446.w2s5f6nr7i7vw4ve@desk>
Date: Mon, 20 Oct 2025 12:44:54 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Jon Kohler <jon@...anix.com>
Cc: 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,
	"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-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out

On Thu, Oct 16, 2025 at 06:12:49PM -0700, Jon Kohler wrote:
> A VMM may not expose ITS_NO or BHI_CTL, so guests cannot rely on those
> bits to determine whether they might be migrated to ITS-affected
> hardware. Rather than depending on a control that may be absent, detect
> ITS-unaffected hosts via a CPU feature that is exclusive to Sapphire
> Rapids and newer processors.

BHI_CTRL is also exclusive to Sapphire Rapids and newer processors. Why
wouldn't it be exposed to guests but BUS_LOCK_DETECT would be?

Not exposing BHI_CTRL has another disadvantage that guests would deploy the
BHB-clear sequence when they could have used cheaper hardware mitigation
for BHI.

> Use X86_FEATURE_BUS_LOCK_DETECT as the canary: it is present on
> Sapphire Rapids+ parts and provides a reliable indicator that the guest
> won't be moved to ITS-affected hardware.

I am puzzled why BUS_LOCK_DETECT is more reliable than BHI_CTRL?