[PATCH AUTOSEL 4.19 073/167] x86/kvmclock: set offset for kvm unstable clock

From: Sasha Levin
Date: Tue Sep 03 2019 - 12:36:07 EST

From: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>

[ Upstream commit b5179ec4187251a751832193693d6e474d3445ac ]

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: 857baa87b642 ("sched/clock: Enable sched clock early")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
arch/x86/kernel/kvmclock.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 013fe3d21dbb3..2ec202cb9dfd4 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -117,12 +117,8 @@ static u64 kvm_sched_clock_read(void)

static inline void kvm_sched_clock_init(bool stable)
- if (!stable) {
- pv_time_ops.sched_clock = kvm_clock_read;
+ if (!stable)
- return;
- }
kvm_sched_clock_offset = kvm_clock_read();
pv_time_ops.sched_clock = kvm_sched_clock_read;