Re: [RESEND PATCH 0/7] Mitigation of "failed to load cpu:cpreg_vmstate_array_len" migration failures

[Eric Auger <eric.auger@redhat.com>]

On 10/16/25 3:59 PM, Eric Auger wrote:
> When migrating ARM guests accross same machines with different host
> kernels we are likely to encounter failures such as:
>
> "failed to load cpu:cpreg_vmstate_array_len"
>
> This is due to the fact KVM exposes a different number of registers
> to qemu on source and destination. When trying to migrate a bigger
> register set to a smaller one, qemu cannot save the CPU state.
>
> For example, recently we faced such kind of situations with:
> - unconditionnal exposure of KVM_REG_ARM_VENDOR_HYP_BMAP_2 FW pseudo
>   register from v6.16 onwards. Causes backward migration failure.
> - removal of unconditionnal exposure of TCR2_EL1, PIRE0_EL1, PIR_EL1
>   from v6.13 onwards. Causes forward migration failure.
>
> This situation is really problematic for distributions which want to
> guarantee forward and backward migration of a given machine type
> between different releases.
>
> This small series tries to address that issue by introducing CPU
> array properties that list the registers to ignore or to fake according
> to the situation. An example is given to illustrate how those props
> could be used to apply compats for machine types supposed to "see" the
> same register set accross various host kernels.
>
> The first patch improves the tracing so that we can quickly detect
> which registers are unexpected and cause the migration failure. Missing
> registers are also traced. Those do not fail migration but their default
> value is kept on the destination.
>
> Then we introduce the infrastructure to handle 'hidden' registers and
> 'fake' registers.
>
> Eric Auger (7):
>   target/arm/machine: Improve traces on register mismatch during
>     migration
>   target/arm/kvm: Introduce the concept of hidden KVM regs
>   target/arm/kvm: Introduce the concept of enforced/fake registers
>   kvm-all: Add the capability to blacklist some KVM regs
>   target/arm/cpu: Implement hide_reg callback()
>   target/arm/kvm: Expose kvm-hidden-regs and kvm-fake-regs properties
>   hw/arm/virt: [DO NOT UPSTREAM] Enforce compatibility with older
>     kernels

Gentle ping.

Any comments on the approach?

Thanks

Eric
>
>  include/hw/core/cpu.h   |  2 ++
>  target/arm/cpu.h        | 42 ++++++++++++++++++++++++
>  accel/kvm/kvm-all.c     | 12 +++++++
>  hw/arm/virt.c           | 19 +++++++++++
>  target/arm/cpu.c        | 12 +++++++
>  target/arm/kvm.c        | 73 ++++++++++++++++++++++++++++++++++++++++-
>  target/arm/machine.c    | 71 +++++++++++++++++++++++++++++++++++----
>  target/arm/trace-events | 11 +++++++
>  8 files changed, 235 insertions(+), 7 deletions(-)
>