Re: [PATCH v2 0/7] xen/arm: Sanitize cpuinfo

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Julien

> On 24 Aug 2021, at 11:01, Julien Grall <julien@xxxxxxx> wrote:
> 
> 
> 
> On 23/08/2021 16:48, Bertrand Marquis wrote:
>> Hi Julien,
> 
> Hi Bertrand,
> 
>> [Keep only arm maintainers in the CC list]
>>> On 23 Aug 2021, at 13:10, Julien Grall <julien@xxxxxxx> wrote:
>>> 
>>> Hi Bertrand,
>>> 
>>> On 23/08/2021 11:32, Bertrand Marquis wrote:
>>>> On arm architecture we might have heterogeneous platforms with different
>>>> types of cores. As a guest can potentialy run on any of those cores we
>>>> have to present them cpu features which are compatible with all cores
>>>> and discard the features which are only available on some cores.
>>> 
>>> Sanitizing the CPU info is important for Xen so it can correctly size the 
>>> P2M, flush the cache... However, I don't think this is going to be 
>>> sufficient to be able to move a vCPU between different type of pCPU.
>>> 
>>> The main hurdle I can see so far is errata handling. Not all the errata can 
>>> be fully handled in Xen so some of them are left to the guest to mitigate.
>> I agree this is something to work on and a problem with the current serie.
>>> 
>>> The errata are usually detected based on the MIDR while the OS is booting. 
>>> IOW, a guest will not be able to know that it needs to handle an errata for 
>>> pCPU B if it only runs on pCPU A.
>> Ack.
>>> 
>>> I don't know yet how this can be solved, but errata are not that uncommon 
>>> on Arm. So until this addressed, we will still need to make sure that vCPUs 
>>> are not migrated between pCPUs with at least a different MIDR.
>>> 
>>> This prevention can be either done manually by pinning the vCPUs or 
>>> implementing the proposal that Dario sent a few years ago (see [1]).
>> My current proposal would be the following:
>> - add a command line option to allow to use all cores on a heterogeneous 
>> platform (different MIDR)
> 
> We already have a command line for heterogenous platform. How about re-using 
> it?

I guess you mean hmp-unsafe which is in fact already mentioning big.LITTLE.
That is what I was planning to use (I should have mentioned it)

> 
>> - taint Xen on this case
>> - keep the feature sanitize as it is as on this case it will create a safer 
>> setup (apart from the errata potential problem)
>> - keep current behaviour if command line option is not passed
>> Having a solution to enable all cores (even if it is unsafe) could still be 
>> a good improvement for development on big.LITTLE
>> platforms or for people knowing how to properly configure the system to 
>> prevent the problems by using properly cpupools so
>> I still think this serie with the proposed changes is still making a lot of 
>> sense.
> 
> +1.
> 
>> I will start looking at a long term solution, maybe automatically create a 
>> cpupools on boot or investigate on the design you provided.
> 
> IIRC, all vCPUs of a domain needs to be in the same cpupool. If your plan is 
> to expose big.LITTLE to the guest, then you would need to look at the design 
> proposal from Dario.

I started to look at Dario idea and I was also thinking we could in fact 
pre-create cpupools for each kind of cores.
I will continue to dig and come up with a design for that because we have some 
use cases which might be more
complex like allowing a guest to actually have both big and little cores in 
which case we need to provide them with
the right cpuinfo and make sure the vcpus are always assigned to the same kind 
of core.

I will work on a v3 with the agreed changes and then write a proposal for the 
next step.

Cheers
Bertrand

> 
> Cheers,
> 
> -- 
> Julien Grall