linux-kernel - Re: [PATCH v4 1/8] x86/bugs: Use VM_CLEAR_CPU

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Message-ID: <aROyvB0kZSQYmCh0@google.com>
Date: Tue, 11 Nov 2025 14:03:40 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v4 1/8] x86/bugs: Use VM_CLEAR_CPU_BUFFERS in VMX as well

On Fri, Nov 07, 2025, Borislav Petkov wrote:
> On Mon, Nov 03, 2025 at 10:18:40AM -0800, Pawan Gupta wrote:
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index 08ed5a2e46a5..2be9be782013 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -321,9 +321,11 @@
> >  #endif
> >  .endm
> >  
> > +/* Primarily used in exit-to-userspace path */
> 
> What does "primarily" mean here?
> 
> $ git grep -w CLEAR_CPU_BUFFERS
> 
> says *only* the kernel->user vector.

How about:

/* If necessary, emit VERW on exit-to-userspace to clear CPU buffers. */
#define CLEAR_CPU_BUFFERS \
	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF

> 
> >  #define CLEAR_CPU_BUFFERS \
> >  	__CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF
> >  
> > +/* For use in KVM */
> 
> That's why the "VM_" prefix is there.

Ya, and this goes away (moved into SVM) by the end of the series.