Re: [PATCH v2 1/8] target/arm/machine: Improve traces on register mismatch during migration

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Nov 18 2025, Eric Auger <eric.auger@redhat.com> wrote:

> Currently when the number of KVM registers exposed by the source is
> larger than the one exposed on the destination, the migration fails
> with: "failed to load cpu:cpreg_vmstate_array_len"
>
> This gives no information about which registers are causing the trouble.
>
> This patch rework the target/arm/machine code so that it becomes

s/rework/reworks/

> able to handle an input stream with a larger set of registers than
> the destination and print useful information about which registers
> are causing the trouble. The migration outcome is unchanged:
> - unexpected registers still will fail the migration
> - missing ones are printed but will not fail the migration, as done today.
>
> The input stream can contain MAX_CPREG_VMSTATE_ANOMALIES(10) extra
> registers compared to what exists on the target.
>
> If there are more registers we will still hit the previous
> "load cpu:cpreg_vmstate_array_len" error.
>
> At most, MAX_CPREG_VMSTATE_ANOMALIES missing registers
> and MAX_CPREG_VMSTATE_ANOMALIES unexpected registers are printed.
>
> Example:
>
> qemu-system-aarch64: kvm_arm_cpu_post_load Missing register in input stream: 0 0x6030000000160003 fw feat reg 3
> qemu-system-aarch64: kvm_arm_cpu_post_load Unexpected register in input stream: 0 0x603000000013c103 op0:3 op1:0 crn:2 crm:0 op2:3
> qemu-system-aarch64: kvm_arm_cpu_post_load Unexpected register in input stream: 1 0x603000000013c512 op0:3 op1:0 crn:10 crm:2 op2:2
> qemu-system-aarch64: kvm_arm_cpu_post_load Unexpected register in input stream: 2 0x603000000013c513 op0:3 op1:0 crn:10 crm:2 op2:3
> qemu-system-aarch64: error while loading state for instance 0x0 of device 'cpu'
> qemu-system-aarch64: load of migration failed: Operation not permitted
>
> With TCG there no user friendly formatting of the faulting

s/there/there is/

> register indexes as with KVM. However the 2 added trace points
> help to identify the culprint indexes.

s/culprint/culprit/

...although "culprint" would make a nice portmanteau :)

>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>
> ---
>
> v1 -> v2:
> - fixed some type in the commit msg
> ---
>  target/arm/cpu.h        |  6 +++++
>  target/arm/kvm.c        | 23 ++++++++++++++++
>  target/arm/machine.c    | 58 ++++++++++++++++++++++++++++++++++++-----
>  target/arm/trace-events |  7 +++++
>  4 files changed, 88 insertions(+), 6 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>