Re: [RESEND PATCH 1/7] target/arm/machine: Improve traces on register mismatch during migration

[Cornelia Huck <cohuck@redhat.com>]

On Thu, Oct 16 2025, Eric Auger <eric.auger@redhat.com> wrote:

More information is really valuable here. I have some nits :)

> Currently whenthe number of KVM registers exposed by the source is

s/whenthe/when the/

> 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 patches rework the target/arm/machine code so that it becomes

s/patches rework/patch 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 print but will not fail the migration, as done today.

s/print/printed/

>
> 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 print.

s/print/printed/

If we really get tons of register discrepancies, I'd expect the reason for
that to be something more obvious, so limiting should be fine.

>
> 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
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> ---
>  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(-)
>
> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
> index bf221e6f97..a7ed3f34f8 100644
> --- a/target/arm/cpu.h
> +++ b/target/arm/cpu.h
> @@ -936,6 +936,12 @@ struct ArchCPU {
>      uint64_t *cpreg_vmstate_values;
>      int32_t cpreg_vmstate_array_len;
>  
> +    #define MAX_CPREG_VMSTATE_ANOMALIES 10
> +    uint64_t cpreg_vmstate_missing_indexes[MAX_CPREG_VMSTATE_ANOMALIES];
> +    int32_t cpreg_vmstate_missing_indexes_array_len;
> +    uint64_t cpreg_vmstate_unexpected_indexes[MAX_CPREG_VMSTATE_ANOMALIES];
> +    int32_t cpreg_vmstate_unexpected_indexes_array_len;

"indices"?

> +
>      DynamicGDBFeatureInfo dyn_sysreg_feature;
>      DynamicGDBFeatureInfo dyn_svereg_feature;
>      DynamicGDBFeatureInfo dyn_smereg_feature;

(...)