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Re: [PATCH v4] xen/arm: avoid overflow when setting vtimer in context switch
- To: Julien Grall <julien@xxxxxxx>
- From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
- Date: Thu, 7 Jul 2022 15:35:52 +0000
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- Cc: Jiamei Xie <Jiamei.Xie@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Wei Chen <Wei.Chen@xxxxxxx>
- Delivery-date: Thu, 07 Jul 2022 15:36:12 +0000
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- Thread-topic: [PATCH v4] xen/arm: avoid overflow when setting vtimer in context switch
Hi Julien,
> On 7 Jul 2022, at 16:33, Julien Grall <julien@xxxxxxx> wrote:
>
> Hi Jiamei,
>
> On 06/07/2022 09:25, Jiamei Xie wrote:
>> virt_vtimer_save() will calculate the next deadline when the vCPU is
>> scheduled out. At the moment, Xen will use the following equation:
>> virt_timer.cval + virt_time_base.offset - boot_count
>> The three values are 64-bit and one (cval) is controlled by domain. In
>> theory, it would be possible that the domain has started a long time
>> after the system boot. So virt_time_base.offset - boot_count may be a
>> large numbers.
>> This means a domain may inadvertently set a cval so the result would
>> overflow. Consequently, the deadline would be set very far in the
>> future. This could result to loss of timer interrupts or the vCPU
>> getting block "forever".
>> One way to solve the problem, would be to separately
>> 1) compute when the domain was created in ns
>> 2) convert cval to ns
>> 3) Add 1 and 2 together
>> The first part of the equation never change (the value is set/known at
>> domain creation). So take the opportunity to store it in domain structure.
>> Signed-off-by: Jiamei Xie <jiamei.xie@xxxxxxx>
>
> Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
>
> The commit message is my own, I would like to Bertrand or Stefano to confirm
> they are happy with it :).
I am ok with it so:
Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
Cheers
Bertrand
>
> Cheers,
>
> --
> Julien Grall
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