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

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

[Borislav Petkov]

From: Borislav Petkov <bp@suse.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@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.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

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

[Sebastian Andrzej Siewior]

On 2019-01-17 13:07:28 [+0100], Borislav Petkov wrote:
> From: Borislav Petkov <bp@suse.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.

makes sense.

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Sebastian

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

[Andy Lutomirski]

On Thu, Jan 17, 2019 at 4:07 AM Borislav Petkov <bp@alien8.de> wrote:
>
> From: Borislav Petkov <bp@suse.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.
>


Reviewed-by: Andy Lutomirski <luto@kernel.org>

[tip:x86/fpu] x86/traps: Have read_cr0() only once in the #NM handler

[tip-bot for Borislav Petkov]

Commit-ID:  ee35b9b9f6d52ba134b4e75442531935f295be7a
Gitweb:     https://git.kernel.org/tip/ee35b9b9f6d52ba134b4e75442531935f295be7a
Author:     Borislav Petkov <bp@suse.de>
AuthorDate: Thu, 17 Jan 2019 13:02:05 +0100
Committer:  Borislav Petkov <bp@suse.de>
CommitDate: Tue, 22 Jan 2019 14:13:35 +0100

x86/traps: Have read_cr0() only once in the #NM handler

... 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@suse.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20190117120728.3811-1-bp@alien8.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);