On 10/01/2017 11:25, Huaitong Han wrote:
> kvmclock_offset is negative, it should be timespec64_add intead of
> timespec64_sub.
>
> kvm->arch.kvmclock_offset = -ktime_get_boot_ns()
No, timespec64_sub is correct (and kvmclock.flat in kvm-unit-tests tests
it). The guest computes CLOCK_REALTIME as pvclock_wall_clock + kvmclock
(see pvclock_read_wallclock in arch/x86/kvm/pvclock.c).
So:
host_boot_time = getboottime64()
pvclock_wall_clock = guest_boot_time
kvmclock_offset = host_boot_time - guest_boot_time
The latter is important: kvmclock_offset is negative because
guest_boot_time > host_boot_time. Then:
pvclock_wall_clock =
= guest_boot_time
= host_boot_time - (host_boot_time - guest_boot_time)
= getboottime64() - kvmclock_offset
Thanks,
Paolo
> Signed-off-by: Huaitong Han <huaitong.han@xxxxxxxxx>
> ---
> arch/x86/kvm/x86.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2f22810..7668a0e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1208,7 +1208,7 @@ static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
>
> if (kvm->arch.kvmclock_offset) {
> struct timespec64 ts = ns_to_timespec64(kvm->arch.kvmclock_offset);
> - boot = timespec64_sub(boot, ts);
> + boot = timespec64_add(boot, ts);
> }
> wc.sec = (u32)boot.tv_sec; /* overflow in 2106 guest time */
> wc.nsec = boot.tv_nsec;
>
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