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

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Message-ID: <20251107190534.GTaQ5C_l_UsZmQR1ph@fat_crate.local>
Date: Fri, 7 Nov 2025 20:05:34 +0100
From: Borislav Petkov <bp@...en8.de>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Sean Christopherson <seanjc@...gle.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 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.

>  #define CLEAR_CPU_BUFFERS \
>  	__CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF
>  
> +/* For use in KVM */

That's why the "VM_" prefix is there.

The comments in arch/x86/include/asm/cpufeatures.h actually already explain
that, you could make them more explicit but let's not sprinkle comments
willy-nilly.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette