Re: [PATCH v6 4/6] xen/cpupool: Create different cpupools at boot time

[Julien Grall <julien@xxxxxxx>]

Hi Luca,

On 08/04/2022 09:45, Luca Fancellu wrote:
> diff --git a/docs/misc/arm/device-tree/cpupools.txt 
> b/docs/misc/arm/device-tree/cpupools.txt
> new file mode 100644
> index 000000000000..40cc8135c66f
> --- /dev/null
> +++ b/docs/misc/arm/device-tree/cpupools.txt
> @@ -0,0 +1,140 @@
> +Boot time cpupools
> +==================
> +
> +When BOOT_TIME_CPUPOOLS is enabled in the Xen configuration, it is possible to
> +create cpupools during boot phase by specifying them in the device tree.
> +ACPI is not supported for this feature.
> +
> +Cpupools specification nodes shall be direct childs of /chosen node.
> +Each cpupool node contains the following properties:
> +
> +- compatible (mandatory)
> +
> +    Must always include the compatiblity string: "xen,cpupool".
> +
> +- cpupool-cpus (mandatory)
> +
> +    Must be a list of device tree phandle to nodes describing cpus (e.g. having
> +    device_type = "cpu"), it can't be empty.
> +
> +- cpupool-sched (optional)
> +
> +    Must be a string having the name of a Xen scheduler. Check the sched=<...>
> +    boot argument for allowed values [1]. When this property is omitted, the 
> Xen
> +    default scheduler will be used.
> +
> +
> +Constraints
> +===========
> +
> +If no cpupools are specified, all cpus will be assigned to one cpupool
> +implicitly created (Pool-0).
> +
> +If cpupools node are specified, but not every cpu brought up by Xen is 
> assigned,
> +all the not assigned cpu will be assigned to an additional cpupool.
> +
> +If a cpu is assigned to a cpupool, but it's not brought up correctly, Xen will
> +stop.
> +
> +The boot cpu must be assigned to Pool-0, so the cpupool containing that core
> +will become Pool-0 automatically.
> +
> +
> +Examples
> +========
> +
> +A system having two types of core, the following device tree specification will
> +instruct Xen to have two cpupools:
> +
> +- The cpupool with id 0 will have 4 cpus assigned.
> +- The cpupool with id 1 will have 2 cpus assigned.
AFAIK, there are no guarantee that Xen will parse cpupool_a first. So it 
would be possible that the ID are inverted here.
This could happen if you want to keep the boot CPU in pool 0 and it is 
not cpu@0 (some bootloaders allows you to change the boot CPU).
Also, here you write "The cpupool with id X" but ...

> +A system having the cpupools specification below will instruct Xen to have 
> three
> +cpupools:
> +
> +- The cpupool Pool-0 will have 2 cpus assigned.
> +- The cpupool Pool-1 will have 2 cpus assigned.
> +- The cpupool Pool-2 will have 2 cpus assigned (created by Xen with all the not
> +  assigned cpus a53_3 and a53_4).
here you write "The cpupool Pool-X". Can you be consistent?

On a separate topic, I think dom0_max_vcpus() needs to be updated to by 
default (i.e when opt_dom0_max_vcpus == 0) the number of vCPUs match the 
number of vCPUs in the cpupool (I think 0) used to created dom0.
Cheers,

--
Julien Grall