Re: [RFC PATCH 0/6] KVM: arm64: Errata management for VM Live migration

[Cornelia Huck <cohuck@redhat.com>]

On Fri, Oct 11 2024, Shameer Kolothum <shameerali.kolothum.thodi@huawei.com> wrote:

> Hi,
>
> On ARM64 platforms most of the errata workarounds are based on CPU
> MIDR/REVIDR values and a number of these workarounds need to be
> implemented by the Guest kernel as well. This creates a problem when
> Guest needs to be migrated to a platform that differs in these
> MIDR/REVIDR values even if the VMM can come up with a common minimum
> feature list for the Guest using the recently introduced "Writable
> ID registers" support.
>
> (This is roughly based on a discussion I had with Marc and Oliver
> at KVM forum. Marc outlined his idea for a solution and this is an
> attempt to implement it. Thanks to both and I take all the blame
> if this is nowhere near what is intended/required)
>
> This RFC proposes a solution to handle the above issue by introducing
> the following,
>
> 1. A new VM IOCTL,
>    KVM_ARM_SET_MIGRN_TARGET_CPUS  _IOW(KVMIO,  0xb7, struct kvm_arm_migrn_cpus)
>    This can be used by the userspace(VMM) to set the target CPUs the
>    Guest will run in its lifetime. See patch #2
> 2. Add hypercall support for Guest kernel to retrieve any migration
>    errata bitmap(ARM_SMCCC_VENDOR_HYP_KVM_MIGRN_ERRATA)
>    The above will return the bitmaps in R0-R3 registers. See patch #4
> 3. The "capability" field in struct arm64_cpu_capabilities is a generated
>    one at present and may get renumbered or reordered. Hence, we can't use
>    this directly for migration errata bitmaps. Instead, introduced
>    "migartion_safe_cap", which has to be set statically for any
>    erratum that needs to be enabled and is safe for migration
>    purposes. See patches 3 & 6.
> 4. Rest of the patches includes the plumbing required to populate the
>    errata bitmap based on the target CPUs set by the VMM and update the
>    system_cap based on it.
>
> ToDos:-
>   -We still need a way to  handle the error in setting the invariant
>    registers(MIDR/REVIDR/AIDR) during Guest migration. Perhaps we can
>    handle it in userspace?
> -  Possibly we could do better to avoid the additional "migartion_safe_cap" use.
>    Suggestions welcome.
>   -There are errata that require more than MIDR/REVIDR, eg: CTR_EL0.
>    How to handle those?
>   -Check for locking requirements if any.
>
> This is lightly tested on a HiSilicon ARM64 platform.
>
> Please take a look and let me know your thoughts.

So, I've only taken a very quick look at it, but IIUC, the idea is for
the VMM to do the following:
- figure out where we want to possibly run
- figure out the least common denominator for writable features
- tell KVM about the possible target cpu resp. the errata it wants
- build a frankencpu via the writable id reg infrastructure, fiddling
  with invariant handling as needed?

(Sorry, it's late in the week :)