linux-kernel - Re: [PATCH v4 2/8] x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

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Message-ID: <DDWH6WN6G64S.22FTEH7M615YJ@google.com>
Date: Fri, 31 Oct 2025 11:37:08 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>
Cc: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v4 2/8] x86/bugs: Decouple ALTERNATIVE usage from VERW
 macro definition

On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> Decouple the use of ALTERNATIVE from the encoding of VERW to clear CPU
> buffers so that KVM can use ALTERNATIVE_2 to handle "always clear buffers"
> and "clear if guest can access host MMIO" in a single statement.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/include/asm/nospec-branch.h | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 08ed5a2e46a5..923ae21cbef1 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -308,24 +308,23 @@
>   * CFLAGS.ZF.
>   * Note: Only the memory operand variant of VERW clears the CPU buffers.
>   */
> -.macro __CLEAR_CPU_BUFFERS feature
>  #ifdef CONFIG_X86_64
> -	ALTERNATIVE "", "verw x86_verw_sel(%rip)", \feature
> +#define CLEAR_CPU_BUFFERS_SEQ	verw x86_verw_sel(%rip)
>  #else
> -	/*
> -	 * In 32bit mode, the memory operand must be a %cs reference. The data
> -	 * segments may not be usable (vm86 mode), and the stack segment may not
> -	 * be flat (ESPFIX32).
> -	 */
> -	ALTERNATIVE "", "verw %cs:x86_verw_sel", \feature
> +/*
> + * In 32bit mode, the memory operand must be a %cs reference. The data segments
> + * may not be usable (vm86 mode), and the stack segment may not be flat (ESPFIX32).
> + */
> +#define CLEAR_CPU_BUFFERS_SEQ	verw %cs:x86_verw_sel
>  #endif
> -.endm
> +
> +#define __CLEAR_CPU_BUFFERS	__stringify(CLEAR_CPU_BUFFERS_SEQ)

Maybe CLEAR_CPU_BUFFERS_SEQ should just be defined as a string in the
first place? But meh, that's a very bikeshed comment. I can see the
aeshetic appeal of the separate __stringify().

Reviewed-by: Brendan Jackman <jackmanb@...gle.com>

>  
>  #define CLEAR_CPU_BUFFERS \
> -	__CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF
> +	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF
>  
>  #define VM_CLEAR_CPU_BUFFERS \
> -	__CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF_VM
> +	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
>  
>  #ifdef CONFIG_X86_64
>  .macro CLEAR_BRANCH_HISTORY