linux-kernel - [PATCH 1/5] x86/asm: Clarify static_cpu

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Date:   Sat, 30 Mar 2019 12:20:18 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Subject: [PATCH 1/5] x86/asm: Clarify static_cpu_has()'s intended use

From: Borislav Petkov <bp@...e.de>

Clarify when one should use static_cpu_has() and when one should use
boot_cpu_has().

Requested-by: Nadav Amit <nadav.amit@...il.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: x86@...nel.org
---
 arch/x86/include/asm/cpufeature.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 30cf12c81db3..1d337c51f7e6 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -156,9 +156,12 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
 #else
 
 /*
- * Static testing of CPU features.  Used the same as boot_cpu_has().
- * These will statically patch the target code for additional
- * performance.
+ * Static testing of CPU features. Used the same as boot_cpu_has(). It
+ * statically patches the target code for additional performance. Use
+ * static_cpu_has() only in fast paths, where every cycle counts. Which
+ * means that the boot_cpu_has() variant is already fast enough for the
+ * majority of cases and you should stick to using it as it is generally
+ * only two instructions: a RIP-relative MOV and a TEST.
  */
 static __always_inline bool _static_cpu_has(u16 bit)
 {
-- 
2.21.0