linux-kernel - [PATCH] x86/traps: Have read_cr0() only once in the #NM handler

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Date:   Thu, 17 Jan 2019 13:07:28 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     X86 ML <x86@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH] x86/traps: Have read_cr0() only once in the #NM handler

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

... instead of twice in the code. In any case, CR0 ends up being read
once anyway:

1. The CONFIG_MATH_EMULATION case does so and exits.
2. The normal case does it once too.

However, read it on function entry instead to make the code even simpler
to follow.

No functional changes.

Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/kernel/traps.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9b7c4ca8f0a7..2684a9d11e66 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -880,12 +880,12 @@ do_spurious_interrupt_bug(struct pt_regs *regs, long error_code)
 dotraplinkage void
 do_device_not_available(struct pt_regs *regs, long error_code)
 {
-	unsigned long cr0;
+	unsigned long cr0 = read_cr0();
 
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
 
 #ifdef CONFIG_MATH_EMULATION
-	if (!boot_cpu_has(X86_FEATURE_FPU) && (read_cr0() & X86_CR0_EM)) {
+	if (!boot_cpu_has(X86_FEATURE_FPU) && (cr0 & X86_CR0_EM)) {
 		struct math_emu_info info = { };
 
 		cond_local_irq_enable(regs);
@@ -897,7 +897,6 @@ do_device_not_available(struct pt_regs *regs, long error_code)
 #endif
 
 	/* This should not happen. */
-	cr0 = read_cr0();
 	if (WARN(cr0 & X86_CR0_TS, "CR0.TS was set")) {
 		/* Try to fix it up and carry on. */
 		write_cr0(cr0 & ~X86_CR0_TS);
-- 
2.19.1