[PATCH] perf/x86: always warn about broken BIOS

[PATCH] perf/x86: always warn about broken BIOS

[Arnd Bergmann]

The intialization function checks for various failure scenarios, but
unfortunately the compiler gets a little confused about the possible
combinations, leading to a false-positive build warning when
-Wmaybe-uninitialized is set:

arch/x86/events/core.c: In function ‘init_hw_perf_events’:
arch/x86/events/core.c:264:3: warning: ‘reg_fail’ may be used uninitialized in this function [-Wmaybe-uninitialized]
arch/x86/events/core.c:264:3: warning: ‘val_fail’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   pr_err(FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n",

It seems reasonable to move the check for the broken BIOS a little
higher in the function, which will lead to the kernel warning about
both the BIOS and and broken hardware if both are faulty, but it
has no effect otherwise at runtime.

Moving it ahead of the check for broken hardware however gets rid
of the compile-time warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/x86/events/core.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 76697acdc507..185d3b27e881 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -228,6 +228,15 @@ static bool check_hw_exists(void)
 	}
 
 	/*
+	 * We still allow the PMU driver to operate:
+	 */
+	if (bios_fail) {
+		pr_cont("Broken BIOS detected, complain to your hardware vendor.\n");
+		pr_err(FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n",
+			      reg_fail, val_fail);
+	}
+
+	/*
 	 * If all the counters are enabled, the below test will always
 	 * fail.  The tools will also become useless in this scenario.
 	 * Just fail and disable the hardware counters.
@@ -252,15 +261,6 @@ static bool check_hw_exists(void)
 	if (ret || val != val_new)
 		goto msr_fail;
 
-	/*
-	 * We still allow the PMU driver to operate:
-	 */
-	if (bios_fail) {
-		pr_cont("Broken BIOS detected, complain to your hardware vendor.\n");
-		pr_err(FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n",
-			      reg_fail, val_fail);
-	}
-
 	return true;
 
 msr_fail:
-- 
2.9.0

Re: [PATCH] perf/x86: always warn about broken BIOS

[Peter Zijlstra]

On Mon, Oct 24, 2016 at 05:26:37PM +0200, Arnd Bergmann wrote:
> The intialization function checks for various failure scenarios, but
> unfortunately the compiler gets a little confused about the possible
> combinations, leading to a false-positive build warning when
> -Wmaybe-uninitialized is set:
> 
> arch/x86/events/core.c: In function ‘init_hw_perf_events’:
> arch/x86/events/core.c:264:3: warning: ‘reg_fail’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> arch/x86/events/core.c:264:3: warning: ‘val_fail’ may be used uninitialized in this function [-Wmaybe-uninitialized]
>    pr_err(FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n",
> 
> It seems reasonable to move the check for the broken BIOS a little
> higher in the function, which will lead to the kernel warning about
> both the BIOS and and broken hardware if both are faulty, but it
> has no effect otherwise at runtime.
> 
> Moving it ahead of the check for broken hardware however gets rid
> of the compile-time warning.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>


Wouldn't something simple like this not be, well, simpler?
---

 arch/x86/events/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index d31735f37ed7..3bc4d76ce0bb 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -190,8 +190,8 @@ static void release_pmc_hardware(void) {}
 
 static bool check_hw_exists(void)
 {
-	u64 val, val_fail, val_new= ~0;
-	int i, reg, reg_fail, ret = 0;
+	u64 val, val_fail = 0, val_new= ~0;
+	int i, reg, reg_fail = 0, ret = 0;
 	int bios_fail = 0;
 	int reg_safe = -1;