linux-kernel - Re: [PATCH v3 3/8] x86, lib: Add WBNOINVD helper functions

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Message-ID: <20250710155824.GA905792@noisy.programming.kicks-ass.net>
Date: Thu, 10 Jul 2025 17:58:24 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Borislav Petkov <bp@...en8.de>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	Kevin Loughlin <kevinloughlin@...gle.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Kai Huang <kai.huang@...el.com>, Ingo Molnar <mingo@...nel.org>,
	Zheyun Shen <szy0127@...u.edu.cn>,
	Mingwei Zhang <mizhang@...gle.com>,
	Francesco Lavra <francescolavra.fl@...il.com>
Subject: Re: [PATCH v3 3/8] x86, lib: Add WBNOINVD helper functions

On Thu, Jul 10, 2025 at 08:55:02AM -0700, Sean Christopherson wrote:

> > So kvm-amd is the SEV stuff, AGPGART is the ancient crap nobody cares
> > about, CCP is more SEV stuff, DRM actually does CLFLUSH loops, but has a
> > WBINVD fallback. i915 is rude and actually does WBINVD. Could they
> > pretty please also do CLFLUSH loops?
> 
> FWIW, doing CLFLUSH in KVM isn't feasible.  In multiple flows, KVM doesn't have
> a valid virtual mapping, and hardware *requires* a WBINVD for at least one of the
> SEV paths.

Yeah, I know. We should give the hardware folks more grief about this
though. If we ever get into the situation of requiring WBINVD, they've
messed up.

> > Anyway, the below seems to survive an allmodconfig.
> > 
> > ---
> > diff --git a/arch/x86/lib/cache-smp.c b/arch/x86/lib/cache-smp.c
> > index c5c60d07308c..ac3cc32a4054 100644
> > --- a/arch/x86/lib/cache-smp.c
> > +++ b/arch/x86/lib/cache-smp.c
> > @@ -12,19 +12,19 @@ void wbinvd_on_cpu(int cpu)
> >  {
> >  	smp_call_function_single(cpu, __wbinvd, NULL, 1);
> >  }
> > -EXPORT_SYMBOL(wbinvd_on_cpu);
> > +EXPORT_SYMBOL_GPL_FOR_MODULES(wbinvd_on_cpu, "kvm-amd,agpgart,ccp,drm,i915");
> 
> Patch 5 of this series drops KVM's homebrewed version of WBINVD-on-CPU, so this
> one at least would need to export the symbol for "kvm" as well.

Ah, sure. At the same time, cpumask_of(cpu) is very cheap.