Re: [PATCH] x86/perf: Fix virtualization sanity check
From: Andre Przywara
Date: Tue Oct 09 2012 - 16:32:35 EST
On 10/09/2012 05:51 PM, Konrad Rzeszutek Wilk wrote:
On Tue, Oct 09, 2012 at 05:38:34PM +0200, Andre Przywara wrote:
In check_hw_exists() we try to detect non-emulated MSR accesses
by writing an arbitrary value into one of the PMU registers
and check if it's value after a readout is still the same.
This algorithm silently assumes that the register does not contain
the magic value already, which is wrong in at least one situation.
Fix the algorithm to really do a read-modify-write cycle. This fixes
a warning under Xen under some circumstances on AMD family 10h CPUs.
The reasons in more details actually sound like a story from
Believe It or Not!:
First you need an AMD family 10h/12h CPU. These do not reset the
PERF_CTR registers on a reboot.
Now you boot bare metal Linux, which goes successfully through this
check, but leaves the magic value of 0xabcd in the register. You
don't use the performance counters, but do a reboot (warm reset).
Then you choose to boot Xen. The check will be triggered with a
recent Linux kernel as Dom0 again, trying to write 0xabcd into the
MSR. Xen silently drops the write (expected), but the subsequent read
will return the value in the register, which just happens to be the
expected magic value. Thus the test misleadingly succeeds, leaving
Is that an oversight in the hypervisor? as in should it disable
access to those MSRs? I thought it disabled to most of them already
unless you give some extra bootup parameters? (cpufreq=dom0 or something
like that).
Probably, it seems like HVM guest get a proper handling of these MSRs,
but Dom0 (and other PV guests) have no special handling. This may be an
addition to the list of things Dom0 is inadvertently allowed to do (or
at least to discover). Found other traces of MCE MSR accesses (and APIC
warnings) just today.
Maybe we should scan the kernel (or dmesg) for those things and handle
them properly in one final(tm) patch.
Btw.: Wei just today sent a patch for Xen to clear the PERF_CTR
registers on boot, so we address this particular problem from both sides.
Regards,
Andre.
the kernel in the belief that the PMU is available. This will trigger
the following message:
[ 0.020294] ------------[ cut here ]------------
[ 0.020311] WARNING: at arch/x86/xen/enlighten.c:730 xen_apic_write+0x15/0x17()
[ 0.020318] Hardware name: empty
[ 0.020323] Modules linked in:
[ 0.020334] Pid: 1, comm: swapper/0 Not tainted 3.3.8 #7
[ 0.020340] Call Trace:
[ 0.020354] [<ffffffff81050379>] warn_slowpath_common+0x80/0x98
[ 0.020369] [<ffffffff810503a6>] warn_slowpath_null+0x15/0x17
[ 0.020378] [<ffffffff810034df>] xen_apic_write+0x15/0x17
[ 0.020392] [<ffffffff8101cb2b>] perf_events_lapic_init+0x2e/0x30
[ 0.020410] [<ffffffff81ee4dd0>] init_hw_perf_events+0x250/0x407
[ 0.020419] [<ffffffff81ee4b80>] ? check_bugs+0x2d/0x2d
[ 0.020430] [<ffffffff81002181>] do_one_initcall+0x7a/0x131
[ 0.020444] [<ffffffff81edbbf9>] kernel_init+0x91/0x15d
[ 0.020456] [<ffffffff817caaa4>] kernel_thread_helper+0x4/0x10
[ 0.020471] [<ffffffff817c347c>] ? retint_restore_args+0x5/0x6
[ 0.020481] [<ffffffff817caaa0>] ? gs_change+0x13/0x13
[ 0.020500] ---[ end trace a7919e7f17c0a725 ]---
The new code will change every of the 16 low bits read from the
register and tries to write and read-back that modified number
from the MSR.
Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
---
arch/x86/kernel/cpu/perf_event.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 915b876..d18b2b8 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -208,12 +208,14 @@ static bool check_hw_exists(void)
}
/*
- * 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.
+ * Read the current value, change it 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;
reg = x86_pmu_event_addr(0);
+ if (rdmsrl_safe(reg, &val))
+ goto msr_fail;
+ val ^= 0xffffUL;
ret = wrmsrl_safe(reg, val);
ret |= rdmsrl_safe(reg, &val_new);
if (ret || val != val_new)
--
1.7.12.1
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