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

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Message-ID: <aHDQersZPA9D8fJb@phenom.ffwll.local>
Date: Fri, 11 Jul 2025 10:51:06 +0200
From: Simona Vetter <simona.vetter@...ll.ch>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>, Sean Christopherson <seanjc@...gle.com>,
	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 05:47:04PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 10, 2025 at 04:37:29PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 10, 2025 at 01:29:02PM +0200, Borislav Petkov wrote:
> > > On Thu, May 22, 2025 at 04:37:27PM -0700, Sean Christopherson wrote:
> > > > diff --git a/arch/x86/lib/cache-smp.c b/arch/x86/lib/cache-smp.c
> > > > index 079c3f3cd32c..1789db5d8825 100644
> > > > --- a/arch/x86/lib/cache-smp.c
> > > > +++ b/arch/x86/lib/cache-smp.c
> > > > @@ -19,3 +19,14 @@ void wbinvd_on_all_cpus(void)
> > > >  	on_each_cpu(__wbinvd, NULL, 1);
> > > >  }
> > > >  EXPORT_SYMBOL(wbinvd_on_all_cpus);
> > > > +
> > > > +static void __wbnoinvd(void *dummy)
> > > > +{
> > > > +	wbnoinvd();
> > > > +}
> > > > +
> > > > +void wbnoinvd_on_all_cpus(void)
> > > > +{
> > > > +	on_each_cpu(__wbnoinvd, NULL, 1);
> > > > +}
> > > > +EXPORT_SYMBOL(wbnoinvd_on_all_cpus);
> > > 
> > > If there's no particular reason for the non-GPL export besides being
> > > consistent with the rest - yes, I did the change for wbinvd_on_all_cpus() but
> > > that was loooong time ago - I'd simply make this export _GPL.
> > 
> > Uhhhh, how about we use this fancy export to known modules only thing
> > for this?
> > 
> > These are typical things we do *NOT* want people to actually use.
> 
> 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?

There's some ancient pentium M where clflush is a no-op (or at least not
getting stuff flushed enough for the gpu to see it), but we still need to
ensure cache coherency with the non-coherent gpu, and wbinvd does the job.
I figured this out with sheer desperation over a decade ago myself,
it's pain.

There shouldn't be any other reason for i915 to wbinvd.
-Sima

> 
> 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");
>  
>  void wbinvd_on_all_cpus(void)
>  {
>  	on_each_cpu(__wbinvd, NULL, 1);
>  }
> -EXPORT_SYMBOL(wbinvd_on_all_cpus);
> +EXPORT_SYMBOL_GPL_FOR_MODULES(wbinvd_on_all_cpus, "kvm-amd,agpgart,ccp,drm,i915,intel-gtt");
>  
>  void wbinvd_on_cpus_mask(struct cpumask *cpus)
>  {
>  	on_each_cpu_mask(cpus, __wbinvd, NULL, 1);
>  }
> -EXPORT_SYMBOL_GPL(wbinvd_on_cpus_mask);
> +EXPORT_SYMBOL_GPL_FOR_MODULES(wbinvd_on_cpus_mask, "kvm,kvm-amd,agpgart,ccp,drm,i915");
>  
>  static void __wbnoinvd(void *dummy)
>  {
> @@ -35,10 +35,10 @@ void wbnoinvd_on_all_cpus(void)
>  {
>  	on_each_cpu(__wbnoinvd, NULL, 1);
>  }
> -EXPORT_SYMBOL_GPL(wbnoinvd_on_all_cpus);
> +EXPORT_SYMBOL_GPL_FOR_MODULES(wbnoinvd_on_all_cpus, "kvm-amd,agpgart,ccp,drm,i915");
>  
>  void wbnoinvd_on_cpus_mask(struct cpumask *cpus)
>  {
>  	on_each_cpu_mask(cpus, __wbnoinvd, NULL, 1);
>  }
> -EXPORT_SYMBOL_GPL(wbnoinvd_on_cpus_mask);
> +EXPORT_SYMBOL_GPL_FOR_MODULES(wbnoinvd_on_cpus_mask, "kvm-amd,agpgart,ccp,drm,i915");

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch