Re: [Xen-devel] [PATCH 6/7] xen/arm: Setup virtual paging for secondary CPUs in non-boot scenario

[Mirela Simonovic <mirela.simonovic@xxxxxxxxxx>]

Hi Julien,


On Wed, Apr 11, 2018 at 5:11 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> Hi,
>
> On 11/04/18 14:19, Mirela Simonovic wrote:
>>
>> In existing code the paging for secondary CPUs is setup only in boot flow.
>> The setup is triggered from start_xen function after all CPUs are brought
>> online. In other words, the initialization of VTCR_EL2 register is done
>> out of the cpu_up/start_secondary control flow. However, the cpu_up flow
>> should be self-contained - it should fully initialize a secondary CPU,
>> because the cpu_up is used not only to bring a secondary CPU online on
>> boot, but also to hotplug a CPU during the system resume.
>> With this patch the setting of paging is triggered from start_secondary
>> function if the current system state is not boot. This way, the paging
>> will be setup in non-boot scenarios, while the setup in boot scenario
>> remains unchanged.
>
>
> I am afraid that this is not correct. You can't assume that value chosen for
> VTCR by Xen at boot will fit this new CPU. So you have to check it is fine
> or park the CPU if there are any issue.
>

This is not a new CPU. This CPU already went through its boot sequence
and it reached the resume point because it does fit the value chosen
for VTCR by Xen.
If it wouldn't fit the chosen value for VTCR it would be parked so it
wouldn't participate in suspend/resume. Please let me know if I
misunderstood your comment.

AFAIU the value chosen by Xen for VTCR config has to be common for all
online CPUs. Since this value is also used in the resume path I
suggest to make global (static in the p2m.c) the 'val' variable which
is currently local in setup_virt_paging() and passed as argument to
setup_virt_paging_one(). Then setup_virt_paging_one() would not
receive an argument.
I need to access this value on resume, so I would call
setup_virt_paging_one() without argument from start_secondary() if the
system state is not boot.
This seems to me a bit cleaner compared to what I submitted in this
patch, but fundamentally the functionality is the same.

Thanks,
Mirela

> For more details have a look at [1].
>
> [1]
> https://lists.xenproject.org/archives/html/xen-devel/2018-01/msg02482.html
>
> Cheers,
>
> --
> Julien Grall

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