linux-kernel - Re: [git-pull -tip] x86:perf

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Date:	Sun, 08 Mar 2009 11:32:39 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jaswinder Singh Rajput <jaswinder@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [git-pull -tip] x86:perf_counter cleanup

On Sun, 2009-03-08 at 15:43 +0530, Jaswinder Singh Rajput wrote:
>  
>  static struct pmc_x86_ops *pmc_amd_init(void)
>  {
> -       u64 old;
> -       int bits;
> -
>         nr_counters_generic = 4;
>         nr_counters_fixed = 0;
>         counter_value_mask = 0x0000FFFFFFFFFFFFULL;



I would rather we try something like the below:

diff --git a/arch/x86/kernel/cpu/perf_counter.c b/arch/x86/kernel/cpu/perf_counter.c
index f585371..ccb4f79 100644
--- a/arch/x86/kernel/cpu/perf_counter.c
+++ b/arch/x86/kernel/cpu/perf_counter.c
@@ -957,12 +957,39 @@ static struct pmc_x86_ops *pmc_amd_init(void)
 	u64 old;
 	int bits;
 
+	pr_info("AMD Performance Monitoring support detected.\n");
+
 	nr_counters_generic = 4;
 	nr_counters_fixed = 0;
-	counter_value_mask = 0x0000FFFFFFFFFFFFULL;
-	counter_value_bits = 48;
+	counter_value_mask = ~0ULL;
 
-	pr_info("AMD Performance Monitoring support detected.\n");
+	rdmsrl(MSR_K7_PERFCTR0, old);
+	wrmsrl(MSR_K7_PERFCTR0, counter_value_mask);
+
+	/*
+	 * Ensure the counter_value_mask write, is completed before
+	 * we try to read it back.
+	 */ 
+	mb();
+
+	/* read the truncated mask */
+	rdmsrl(MSR_K7_PERFCTR0, counter_value_mask);
+	wrmsrl(MSR_K7_PERFCTR0, old);
+
+	bits = 32 + fls(counter_value_mask >> 32);
+	if (bits == 32)
+		bits = fls((u32)counter_value_mask);
+	counter_value_bits = bits;
+
+	if (counter_value_bits != 48 ||
+	    counter_value_mask != ((1ULL << counter_value_bits) - 1)) {
+		printk(KERN_ERR "Bad cpu! speculated through full memory barrier!\n");
+		printk(KERN_ERR " ... old value: %llx\n", old);
+		printk(KERN_ERR " ... new value: %llx\n", counter_value_mask);
+
+		counter_value_bits = 48;
+		counter_value_mask = 0x0000FFFFFFFFFFFFULL;
+	}
 
 	return &pmc_amd_ops;
 }

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