On 26/01/19 18:49, Pavel Tatashin wrote:
> 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")
>
> Reported-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@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 e811d4d1c824..d908a37bf3f3 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -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;
>
>
Queued, thanks.
Pa
