Re: [BUG] Migration hv_time rollback

[Vitaly Kuznetsov]

Antoine Damhet <antoine.damhet@blade-group.com> writes:

> On Wed, Sep 16, 2020 at 12:29:56PM +0100, Dr. David Alan Gilbert wrote:
>> cc'ing in Vitaly who knows about the hv stuff.
>
> Thanks
>
>> 
>> * Antoine Damhet (antoine.damhet@blade-group.com) wrote:
>> > Hi,
>> > 
>> > We are experiencing timestamp rollbacks during live-migration of
>> > Windows 10 guests with the following qemu configuration (linux 5.4.46
>> > and qemu master):
>> > ```
>> > $ qemu-system-x86_64 -enable-kvm -cpu host,kvm=off,hv_time [...]
>> > ```
>> 
>> How big a jump are you seeing, and how did you notice it in the guest?
>
> I'm seeing jumps of about the guest uptime (indicating a reset of the
> counter). It's expected because we won't call `KVM_SET_CLOCK` to
> restore any value.
>
> We first noticed it because after some migrations `dwm.exe` crashes with
> the "(NTSTATUS) 0x8898009b - QueryPerformanceCounter returned a time in
> the past." error code.
>
> I can also confirm the following hack makes the behavior disappear:
>
> ```
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 64283358f9..f334bdf35f 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -332,11 +332,7 @@ void kvmclock_create(void)
>  {
>      X86CPU *cpu = X86_CPU(first_cpu);
>
> -    if (kvm_enabled() &&
> -        cpu->env.features[FEAT_KVM] & ((1ULL << KVM_FEATURE_CLOCKSOURCE) |
> -                                       (1ULL << KVM_FEATURE_CLOCKSOURCE2))) {
> -        sysbus_create_simple(TYPE_KVM_CLOCK, -1, NULL);
> -    }
> +    sysbus_create_simple(TYPE_KVM_CLOCK, -1, NULL);
>  }
>


Oh, I think I see what's going on. When you add 'kvm=off'
cpu->env.features[FEAT_KVM] is reset (see x86_cpu_expand_features()) so
kvmclock QEMU device is not created and nobody calls KVM_SET_CLOCK on
migration.

In case we really want to support 'kvm=off' I think we can add Hyper-V
features check here along with KVM, this should do the job.

-- 
Vitaly