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

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 10, 2025, 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?

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.

> 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.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1d0e9180148d..d63a2c27e058 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4965,11 +4965,6 @@ long kvm_arch_dev_ioctl(struct file *filp,
 	return r;
 }
 
-static void wbinvd_ipi(void *garbage)
-{
-	wbinvd();
-}
-
 static bool need_emulate_wbinvd(struct kvm_vcpu *vcpu)
 {
 	return kvm_arch_has_noncoherent_dma(vcpu->kvm);
@@ -4991,8 +4986,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		if (kvm_x86_call(has_wbinvd_exit)())
 			cpumask_set_cpu(cpu, vcpu->arch.wbinvd_dirty_mask);
 		else if (vcpu->cpu != -1 && vcpu->cpu != cpu)
-			smp_call_function_single(vcpu->cpu,
-					wbinvd_ipi, NULL, 1);
+			wbinvd_on_cpu(vcpu->cpu);
 	}
 
 	kvm_x86_call(vcpu_load)(vcpu, cpu);
-- 

>  
>  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");