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

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Message-ID: <20251031003040.3491385-3-seanjc@google.com>
Date: Thu, 30 Oct 2025 17:30:34 -0700
From: Sean Christopherson <seanjc@...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: [PATCH v4 2/8] x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

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
 
 #ifdef CONFIG_X86_64
 .macro CLEAR_BRANCH_HISTORY
-- 
2.51.1.930.gacf6e81ea2-goog