linux-kernel - MMIO and VERW

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Message-ID: <20250318141659.GDZ9mAWwa3dkQDHkCk@fat_crate.local>
Date: Tue, 18 Mar 2025 15:16:59 +0100
From: Borislav Petkov <bp@...en8.de>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: David Kaplan <david.kaplan@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	Brendan Jackman <jackmanb@...gle.com>,
	Derek Manwaring <derekmn@...zon.com>
Subject: MMIO and VERW

Carving this thing out into a separate thread:

On Thu, Mar 13, 2025 at 12:26:06PM -0700, Pawan Gupta wrote:
> On Thu, Mar 13, 2025 at 10:36:17AM +0100, Borislav Petkov wrote:
> > I'd expect to see:
> > 
> > 	if (mmio_mitigation == MMIO_MITIGATION_AUTO) {
> >                 mmio_mitigation = MMIO_MITIGATION_VERW;
> > 		verw_mitigation_selected = true;
> > 	}
> > 
> >         if (boot_cpu_has_bug(X86_BUG_MDS) || taa_vulnerable())
> >                 verw_mitigation_selected = true;
> > 
> > because the above branch already selected MMIO_MITIGATION_VERW so we might as
> > well set verw_mitigation_selected, right?
> 
> There is a subtle difference between setting verw_mitigation_selected and
> MMIO_MITIGATION_VERW. The former is a system-wide switch that indicates
> VERW is needed at both kernel-exit and VMenter. MMIO Stale Data is
> different from other VERW based mitigations because it only requires VERW
> at VMenter, when not affected by MDS/TAA. So, turning the system-wide knob
> here would be wrong.

Realistically speaking, do we have a machine where you *only* enable VERW on
VMENTER?

I'm not talking about some experimentation scenario where one measures which
mitigations cost how much.

Do we have a real-life hw configuration where the *only* VERW mitigation
needed is at VMENTER because that machine is affected *only* by MMIO and no
other VERW-based mitigation is needed?

Because if not, we might as well drop that too-special distinction and
simplify that maze of nasty conditional spaghetti...

Hmmm?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette