[ 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@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 Signed-off-by: Sasha Levin sashal@kernel.org --- 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) clear_sched_clock_stable(); - return; - } - kvm_sched_clock_offset = kvm_clock_read(); pv_time_ops.sched_clock = kvm_sched_clock_read;