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(-)
>
