Re: [PATCH 3/7] KVM: arm64: Only reset vCPU-scoped feature ID regs once

Sebastian Ott
2024-05-13T13:31:12Z

> The general expecation with feature ID registers is that they're 'reset'
^
expectation

> exactly once by KVM for the lifetime of a vCPU/VM, such that any
> userspace changes to the CPU features / identity are honored after a
> vCPU gets reset (e.g. PSCI_ON).
>
> KVM handles what it calls VM-scoped feature ID registers correctly, but
> feature ID registers local to a vCPU (CLIDR_EL1, MPIDR_EL1) get wiped
> after every reset. What's especially concerning is that a
> potentially-changing MPIDR_EL1 breaks MPIDR compression for indexing
> mpidr_data, as the mask of useful bits to build the index could change.
>
> This is absolutely no good. Avoid resetting vCPU feature ID registers
> more than once.

Reviewed-by: Sebastian Ott <sebott@redhat.com>