Re: perf hw in kexeced kernel broken in tip

Peter Zijlstra
2010-12-08T14:59:29Z

On Wed, 2010-12-08 at 15:20 +0100, Peter Zijlstra wrote:

> > I wonder if you should reverse these checks. If the bios has the perf
> > counter enabled, there might be a high chance that it fails the first
> > check and never gets to the actually bios checks.
>
> Ah, good point.

Something like so..

---
Subject: perf, x86: Detect broken BIOSes
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Wed Dec 08 15:56:23 CET 2010

Some BIOSes use PMU resources, this is a bug.

Try to detect this, warn about it, and further refuse to touch the
PMU ourselves.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
Index: linux-2.6/arch/x86/kernel/cpu/perf_event.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/cpu/perf_event.c
+++ linux-2.6/arch/x86/kernel/cpu/perf_event.c
@@ -375,15 +375,51 @@ static void release_pmc_hardware(void) {
static bool check_hw_exists(void)
{
u64 val, val_new = 0;
- int ret = 0;
+ int i, reg, ret = 0;

+ /*
+ * Check to see if the BIOS enabled any of the counters, if so
+ * complain and bail.
+ */
+ for (i = 0; i < x86_pmu.num_counters; i++) {
+ reg = x86_pmu.eventsel + i;
+ ret = rdmsrl_safe(reg, &val);
+ if (ret)
+ goto msr_fail;
+ if (val & ARCH_PERFMON_EVENTSEL_ENABLE)
+ goto bios_fail;
+ }
+
+ for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
+ reg = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
+ ret = rdmsrl_safe(reg, &val);
+ if (ret)
+ goto msr_fail;
+ if (val & (0x03 << i*4))
+ goto bios_fail;
+ }
+
+ /*
+ * Now write a value and read it back to see if it matches,
+ * this is needed to detect certain hardware emulators (qemu/kvm)
+ * that don't trap on the MSR access and always return 0s.
+ */
val = 0xabcdUL;
- ret |= checking_wrmsrl(x86_pmu.perfctr, val);
+ ret = checking_wrmsrl(x86_pmu.perfctr, val);
ret |= rdmsrl_safe(x86_pmu.perfctr, &val_new);
if (ret || val != val_new)
- return false;
+ goto msr_fail;

return true;
+
+bios_fail:
+ printk(KERN_CONT "Broken BIOS detected, software events only.\n");
+ printk(KERN_ERR FW_BUG "invalid MSR: %x=%Lx\n", reg, val);
+ return false;
+
+msr_fail:
+ printk(KERN_CONT "Broken PMU hardware detected, software events only.\n");
+ return false;
}

static void reserve_ds_buffers(void);
@@ -1378,10 +1414,8 @@ int __init init_hw_perf_events(void)
pmu_check_apic();

/* sanity check that the hardware exists or is emulated */
- if (!check_hw_exists()) {
- pr_cont("Broken PMU hardware detected, software events only.\n");
+ if (!check_hw_exists())
return 0;
- }

pr_cont("%s PMU driver.\n", x86_pmu.name);