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

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.

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.

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]).


[1] https://lists.xenproject.org/archives/html/xen-devel/2016-12/msg00826.html

Julien Grall



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