Re: [PATCH v4] xen/arm: avoid overflow when setting vtimer in context switch

[Julien Grall <julien@xxxxxxx>]

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 :).
Cheers,

--
Julien Grall