Re: [PATCH v2 1/1] xen: update system time immediately when VCPUOP_register_vcpu_info

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Jan Beulich writes ("Re: [PATCH v2 1/1] xen: update system time immediately 
when VCPUOP_register_vcpu_info"):
> On 25.10.2021 19:35, Dongli Zhang wrote:
> > The guest may access the pv vcpu_time_info immediately after
> > VCPUOP_register_vcpu_info. This is to borrow the idea of
> > VCPUOP_register_vcpu_time_memory_area, where the
> > force_update_vcpu_system_time() is called immediately when the new memory
> > area is registered.
> > 
> > Otherwise, we may observe clock drift at the VM side if the VM accesses
> > the clocksource immediately after VCPUOP_register_vcpu_info().
> > 
> > Reference:
> > https://lists.xenproject.org/archives/html/xen-devel/2021-10/msg00571.html
> > Cc: Joe Jin <joe.jin@xxxxxxxxxx>
> > Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> 
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Ian - any thoughts towards 4.16 here either way?

This looks like a bugfix to me, and the diff is certainly small.  I am
positively inclined.  I would like to know what the risks are.
Stefano says this does nothing on ARM so the risk would be to x86.
Can you advise ?

Thanks,
Ian.