On 09/14/2010 12:26 PM, Jan Kiszka wrote:
Am 14.09.2010 21:32, Zachary Amsden wrote:
On 09/14/2010 12:40 AM, Jan Kiszka wrote:
Am 14.09.2010 11:27, Avi Kivity wrote:
On 09/14/2010 11:10 AM, Jan Kiszka wrote:
Am 20.08.2010 10:07, Zachary Amsden wrote:
When CPUs with unstable TSCs enter deep C-state, TSC may stop
running. This causes us to require resynchronization. Since
we can't tell when this may potentially happen, we assume the
worst by forcing re-compensation for it at every point the VCPU
task is descheduled.
Signed-off-by: Zachary Amsden<zamsden@xxxxxxxxxx>
---
arch/x86/kvm/x86.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7fc4a55..52b6c21 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1866,7 +1866,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu
*vcpu, int cpu)
}
kvm_x86_ops->vcpu_load(vcpu, cpu);
- if (unlikely(vcpu->cpu != cpu)) {
+ if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
/* Make sure TSC doesn't go backwards */
s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
native_read_tsc() - vcpu->arch.last_host_tsc;
For yet unknown reason, this commit breaks Linux guests here if they
are
started with only a single VCPU. They hang during boot, obviously no
longer receiving interrupts.
I'm using kvm-kmod against a 2.6.34 host kernel, so this may be a side
effect of the wrapping, though I cannot imagine how.
Anyone any ideas?
Most likely, time went backwards, and some 'future - past' calculation
resulted in a negative sleep value which was then interpreted as
unsigned and resulted in a 2342525634 year sleep.
Looks like that's the case on first glance at the apic state.
This compensation effectively nulls the delta between current and last TSC:
if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
/* Make sure TSC doesn't go backwards */
s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
native_read_tsc() -
vcpu->arch.last_host_tsc;
if (tsc_delta< 0)
mark_tsc_unstable("KVM discovered backwards TSC");
if (check_tsc_unstable())
kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
kvm_migrate_timers(vcpu);
vcpu->cpu = cpu;
If TSC has advanced quite a bit due to a TSC jump during sleep(*), it
will adjust the offset backwards to compensate; similarly, if it has
gone backwards, it will advance the offset.
In neither case should the visible TSC go backwards, assuming
last_host_tsc is recorded properly, and so kvmclock should be similarly
unaffected.
Perhaps the guest is more intelligent than we hope, and is comparing two
different clocks: kvmclock or TSC with the rate of PIT interrupts. This
could result in negative arithmetic begin interpreted as unsigned. Are
you using PIT interrupt reinjection on this guest or passing
-no-kvm-pit-reinjection?
Does your guest use kvmclock, tsc, or some other time source?
A kernel that has kvmclock support even hangs in SMP mode. The others
pick hpet or acpi_pm. TSC is considered unstable.
SMP mode here has always and will always be unreliable. Are you running
on an Intel or AMD CPU? The origin of this code comes from a workaround
for (*) in vendor-specific code, and perhaps it is inappropriate for both.
I'm on a fairly new Intel i7 (M 620). And I accidentally rebooted my box
a few hours ago. Well, the issue is gone now...
So I looked into the system logs and found this:
[18446744053.434939] PM: resume of devices complete after 4379.595 msecs
[18446744053.457133] PM: Finishing wakeup.
[18446744053.457135] Restarting tasks ...
[ 0.000999] Marking TSC unstable due to KVM discovered backwards TSC
[270103.974668] done.
From that point on the box was on hpet, including the time I did the
failing tests this morning. The kvm-kmod version loaded at this point
was based on kvm.git df549cfc.
But my /proc/cpuinfo claims "constant_tsc", and Linux is generally happy
with using it as clock source. Does this tell you anything?
Yes, quite a bit.
It's possible that marking the TSC unstable with an actively running VM
causes a boundary condition that I had not accounted for. It's also
possible that the clocksource switch triggered some bad behavior.
This suggests two debugging techniques: I can manually switch the
clocksource, and I can also load a module which does nothing other than
mark the TSC unstable. Failing that, we can investigate PM suspend /
resume for possible issues.
I'll try this on my Intel boxes to see what happens.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
