Hi Julien, [Keep only arm maintainers in the CC list]
> On 23 Aug 2021, at 13:10, Julien Grall <[email protected]> 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) - 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. I will start looking at a long term solution, maybe automatically create a cpupools on boot or investigate on the design you provided. Please give me your view on this. Kind regards Bertrand > > Cheers, > > [1] https://lists.xenproject.org/archives/html/xen-devel/2016-12/msg00826.html > > -- > Julien Grall
