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

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Message-ID: <20251101041324.k2crtjvwqaxhkasr@desk>
Date: Fri, 31 Oct 2025 21:13:24 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: 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>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v4 2/8] x86/bugs: Decouple ALTERNATIVE usage from VERW
 macro definition

On Thu, Oct 30, 2025 at 05:30:34PM -0700, 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)
>  
>  #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

Sorry nitpicking, we have too many "CLEAR_CPU_BUF" in these macros, can we
avoid adding CLEAR_CPU_BUFFERS_SEQ?

Or better yet, can we name the actual instruction define to VERW_SEQ, so as
to differentiate it from the ALTERNATIVE defines?

---
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 4cf347732ec1..16b957382224 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -309,23 +309,21 @@
  * Note: Only the memory operand variant of VERW clears the CPU buffers.
  */
 #ifdef CONFIG_X86_64
-#define CLEAR_CPU_BUFFERS_SEQ	verw x86_verw_sel(%rip)
+#define VERW_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).
  */
-#define CLEAR_CPU_BUFFERS_SEQ	verw %cs:x86_verw_sel
+#define VERW_SEQ	verw %cs:x86_verw_sel
 #endif
 
-#define __CLEAR_CPU_BUFFERS	__stringify(CLEAR_CPU_BUFFERS_SEQ)
-
 /* Primarily used in exit-to-userspace path */
 #define CLEAR_CPU_BUFFERS \
-	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF
+	ALTERNATIVE "", __stringify(VERW_SEQ), X86_FEATURE_CLEAR_CPU_BUF
 
 #define VM_CLEAR_CPU_BUFFERS \
-	ALTERNATIVE "", __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM
+	ALTERNATIVE "", __stringify(VERW_SEQ), X86_FEATURE_CLEAR_CPU_BUF_VM
 
 #ifdef CONFIG_X86_64
 .macro CLEAR_BRANCH_HISTORY