Commit 4407204c authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar

perf, x86: Detect broken BIOSes that corrupt the PMU

Some BIOSes use PMU resources, which can cause various bugs:

 - Non-working or erratic PMU based statistics - the PMU can end up
   counting the wrong thing, resulting in misleading statistics

 - Profiling can stop working or it can profile the wrong thing

 - A non-working or erratic NMI watchdog that cannot be relied on

 - The kernel may disturb whatever thing the BIOS tries to use the
   PMU for - possibly causing hardware malfunction in extreme cases.

 - ... and other forms of potential misbehavior

Various forms of such misbehavior has been observed in practice - there are
BIOSes that just corrupt the PMU state, consequences be damned.

The PMU is a CPU resource that is handled by the kernel and the BIOS
stealing+corrupting it is not acceptable nor robust, so we detect it,
warn about it and further refuse to touch the PMU ourselves.
Signed-off-by: default avatarPeter Zijlstra <>
Cc: Jason Wessel <>
Cc: Don Zickus <>
Cc: Linus Torvalds <>
Cc: Thomas Gleixner <>
Cc: "H. Peter Anvin" <>
LKML-Reference: <new-submission>
Signed-off-by: default avatarIngo Molnar <>
parent 006b20fe
......@@ -375,15 +375,53 @@ 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;
goto bios_fail;
if (x86_pmu.num_counters_fixed) {
ret = rdmsrl_safe(reg, &val);
if (ret)
goto msr_fail;
for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
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;
printk(KERN_CONT "Broken BIOS detected, using software events only.\n");
printk(KERN_ERR FW_BUG "the BIOS has corrupted hw-PMU resources (MSR %x is %Lx)\n", reg, val);
return false;
printk(KERN_CONT "Broken PMU hardware detected, using software events only.\n");
return false;
static void reserve_ds_buffers(void);
......@@ -1378,10 +1416,8 @@ int __init init_hw_perf_events(void)
/* 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",;
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