linux-kernel - [PATCH v3 2/6] x86/bugs: Use SBPB in __write

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Message-ID: <df47d38d252b5825bc86afaf0d021b016286bf06.1743617897.git.jpoimboe@kernel.org>
Date: Wed,  2 Apr 2025 11:19:19 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	amit@...nel.org,
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Subject: [PATCH v3 2/6] x86/bugs: Use SBPB in __write_ibpb() if applicable

__write_ibpb() does IBPB, which (among other things) flushes branch type
predictions on AMD.  If the CPU has SRSO_NO, or if the SRSO mitigation
has been disabled, branch type flushing isn't needed, in which case the
lighter-weight SBPB can be used.

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 arch/x86/entry/entry.S     | 2 +-
 arch/x86/kernel/cpu/bugs.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
index 3a53319988b9..a5b421ec19c0 100644
--- a/arch/x86/entry/entry.S
+++ b/arch/x86/entry/entry.S
@@ -21,7 +21,7 @@
 SYM_FUNC_START(__write_ibpb)
 	ANNOTATE_NOENDBR
 	movl	$MSR_IA32_PRED_CMD, %ecx
-	movl	$PRED_CMD_IBPB, %eax
+	movl	_ASM_RIP(x86_pred_cmd), %eax
 	xorl	%edx, %edx
 	wrmsr
 
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 310cb3f7139c..c8b8dc829046 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -58,7 +58,7 @@ EXPORT_SYMBOL_GPL(x86_spec_ctrl_base);
 DEFINE_PER_CPU(u64, x86_spec_ctrl_current);
 EXPORT_PER_CPU_SYMBOL_GPL(x86_spec_ctrl_current);
 
-u64 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
+u32 x86_pred_cmd __ro_after_init = PRED_CMD_IBPB;
 EXPORT_SYMBOL_GPL(x86_pred_cmd);
 
 static u64 __ro_after_init x86_arch_cap_msr;
-- 
2.48.1