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Re: [PATCH v2 1/1] xen: update system time immediately when VCPUOP_register_vcpu_info
- To: Ian Jackson <iwj@xxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 2 Nov 2021 17:36:36 +0100
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- Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>, sstabellini@xxxxxxxxxx, julien@xxxxxxx, Volodymyr_Babchuk@xxxxxxxx, andrew.cooper3@xxxxxxxxxx, george.dunlap@xxxxxxxxxx, wl@xxxxxxx, joe.jin@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 02 Nov 2021 16:37:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.11.2021 17:00, Ian Jackson wrote:
> 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 ?
I don't see any noteworthy risks - a call to a function gets added
in a 2nd place; the function itself has been working fine for years,
and it is fine to be used in this new context.
Jan
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