linux-kernel - Re: [PATCH v2 7/8] x86, lib: Add wbinvd and wbnoinvd helpers to target multiple CPUs

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Message-ID: <aCtWM63FyQKMJzqE@google.com>
Date: Mon, 19 May 2025 09:02:59 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	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, 
	Zheyun Shen <szy0127@...u.edu.cn>, Tom Lendacky <thomas.lendacky@....com>, 
	Kevin Loughlin <kevinloughlin@...gle.com>, Kai Huang <kai.huang@...el.com>, 
	Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v2 7/8] x86, lib: Add wbinvd and wbnoinvd helpers to
 target multiple CPUs

On Sat, May 17, 2025, Ingo Molnar wrote:
> 
> * Sean Christopherson <seanjc@...gle.com> wrote:
> 
> > From: Zheyun Shen <szy0127@...u.edu.cn>
> > 
> > Extract KVM's open-coded calls to do writeback caches on multiple CPUs to
> > common library helpers for both WBINVD and WBNOINVD (KVM will use both).
> > Put the onus on the caller to check for a non-empty mask to simplify the
> > SMP=n implementation, e.g. so that it doesn't need to check that the one
> > and only CPU in the system is present in the mask.
> > 
> > Signed-off-by: Zheyun Shen <szy0127@...u.edu.cn>
> > Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
> > Link: https://lore.kernel.org/r/20250128015345.7929-2-szy0127@sjtu.edu.cn
> > [sean: move to lib, add SMP=n helpers, clarify usage]
> > Acked-by: Kai Huang <kai.huang@...el.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/include/asm/smp.h | 12 ++++++++++++
> >  arch/x86/kvm/x86.c         |  8 +-------
> >  arch/x86/lib/cache-smp.c   | 12 ++++++++++++
> >  3 files changed, 25 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
> > index e08f1ae25401..fe98e021f7f8 100644
> > --- a/arch/x86/include/asm/smp.h
> > +++ b/arch/x86/include/asm/smp.h
> > @@ -113,7 +113,9 @@ void native_play_dead(void);
> >  void play_dead_common(void);
> >  void wbinvd_on_cpu(int cpu);
> >  void wbinvd_on_all_cpus(void);
> > +void wbinvd_on_many_cpus(struct cpumask *cpus);
> >  void wbnoinvd_on_all_cpus(void);
> > +void wbnoinvd_on_many_cpus(struct cpumask *cpus);
> 
> Let's go with the _on_cpumask() suffix:
> 
>     void wbinvd_on_cpu(int cpu);
>    +void wbinvd_on_cpumask(struct cpumask *cpus);
>     void wbinvd_on_all_cpus(void);
> 
> And the wb*invd_all_cpus() methods should probably be inlined wrappers 
> with -1 as the cpumask, or so - not two separate functions?

Using two separate functions allows _on_all_cpus() to defer the mask generation
to on_each_cpu(), i.e. avoids having to duplicate the passing of cpu_online_mask.
IMO, duplicating passing __wbinvd is preferable to duplicating the use of
cpu_online_mask.
 
> In fact it would be nice to have the DRM preparatory patch and all the 
> x86 patches at the beginning of the next version of the series, so 
> those 4 patches can be applied to the x86 tree. Can make it a separate 
> permanent branch based on v6.15-rc6/rc7.

Can do, assuming there's no lurking dependency I'm missing.