[PATCH v5 3/9] x86/bugs: Decouple ALTERNATIVE usage from VERW macro definition

Sean Christopherson
2025-11-13T23:37:54Z

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.

Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/nospec-branch.h | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 08ed5a2e46a5..8b4885a1b2ef 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -308,24 +308,24 @@
* 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 VERW 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 VERW verw %cs:x86_verw_sel
#endif
-.endm

+#define __CLEAR_CPU_BUFFERS __stringify(VERW)
+
+/* If necessary, emit VERW on exit-to-userspace to clear CPU buffers. */
#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
--
2.52.0.rc1.455.g30608eb744-goog