Re: [RFC PATCH 0/2] Boot time cpupools

[Julien Grall <julien@xxxxxxx>]

Hi Luca,

On 17/11/2021 09:57, Luca Fancellu wrote:
> Currently Xen creates a default cpupool0 that contains all the cpu brought up
> during boot and it assumes that the platform has only one kind of CPU.
> This assumption does not hold on big.LITTLE platform, but putting different
> type of CPU in the same cpupool can result in instability and security issues
> for the domains running on the pool.
I agree that you can't move a LITTLE vCPU to a big pCPU. However...

> For this reason this serie introduces an architecture specific way to create
> different cpupool at boot time, this is particularly useful on ARM big.LITTLE
> platform where there might be the need to have different cpupools for each type
> of core, but also systems using NUMA can have different cpu pool for each node.
... from my understanding, all the vCPUs of a domain have to be in the 
same cpupool. So with this approach it is not possible:
   1) to have a mix of LITTLE and big vCPUs in the domain
   2) to create a domain spanning across two NUMA nodes

So I think we need to make sure that any solutions we go through will 
not prevent us to implement those setups.
I can see two options here:
  1) Allowing a domain vCPUs to be on a different cpupool
  2) Introducing CPU class (see [1])

I can't remember why Dario suggested 2) rather than 1) in the past. 
@Dario, do you remember it?
Cheers,

[1] https://lore.kernel.org/xen-devel/1481135379.3445.142.camel@xxxxxxxxxx/

> To test this serie, the hmp_unsafe Xen boot argument is passed to allow Xen
> starting different CPUs from the boot core.
>
> Luca Fancellu (2):
>    xen/cpupool: Create different cpupools at boot time
>    tools/cpupools: Give a name to unnamed cpupools
>
>   tools/libs/light/libxl_utils.c | 13 ++++--
>   xen/arch/arm/Kconfig           | 15 ++++++
>   xen/arch/arm/Makefile          |  1 +
>   xen/arch/arm/cpupool.c         | 84 ++++++++++++++++++++++++++++++++++
>   xen/common/sched/cpupool.c     |  5 +-
>   xen/include/xen/cpupool.h      | 30 ++++++++++++
>   6 files changed, 142 insertions(+), 6 deletions(-)
>   create mode 100644 xen/arch/arm/cpupool.c
>   create mode 100644 xen/include/xen/cpupool.h
--
Julien Grall