From 1d60902a5448b2bd55806c5883965a03c7e1e309 Mon Sep 17 00:00:00 2001 From: Pavel Tatashin Date: Sat, 26 Jan 2019 12:49:56 -0500 Subject: [PATCH] x86/kvmclock: set offset for kvm unstable clock [ 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 Signed-off-by: Pavel Tatashin Signed-off-by: Paolo Bonzini Signed-off-by: Sasha Levin --- 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 013fe3d..2ec202c 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; -- 2.7.4