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x86/kvmclock: set offset for kvm unstable clock

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VMs may show incorrect uptime and dmesg printk offsets on hypervisors with
unstable clock. The problem is produced when VM is rebooted without exiting
from qemu.

The fix is to calculate clock offset not only for stable clock but for
unstable clock as well, and use kvm_sched_clock_read() which substracts
the offset for both clocks.

This is safe, because pvclock_clocksource_read() does the right thing and
makes sure that clock always goes forward, so once offset is calculated
with unstable clock, we won't get new reads that are smaller than offset,
and thus won't get negative results.

Thank you Jon DeVree for helping to reproduce this issue.

Fixes: 857baa87b6 ("sched/clock: Enable sched clock early")
Cc: stable@vger.kernel.org
Reported-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Pavel Tatashin 2019-01-26 12:49:56 -05:00 committed by Paolo Bonzini
parent 4f44c4eec5
commit b5179ec418
1 changed files with 1 additions and 5 deletions
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6
arch/x86/kernel/kvmclock.c
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@ -104,12 +104,8 @@ static u64 kvm_sched_clock_read(void)
static inline void kvm_sched_clock_init(bool stable)
{
if (!stable) {
pv_ops.time.sched_clock = kvm_clock_read;
if (!stable)
clear_sched_clock_stable();
return;
}
kvm_sched_clock_offset = kvm_clock_read();
pv_ops.time.sched_clock = kvm_sched_clock_read;