Re: [PATCH RFC] arm/kvm: report registers we failed to set

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Aug 12 2025, Eric Auger <eric.auger@redhat.com> wrote:

> Hi Connie,
>
> On 7/21/25 6:19 PM, Cornelia Huck wrote:
>> If we fail migration because of a mismatch of some registers between
>> source and destination, the error message is not very informative:
>>
>> qemu-system-aarch64: error while loading state for instance 0x0 ofdevice 'cpu'
>> qemu-system-aarch64: Failed to put registers after init: Invalid argument
>>
>> At least try to give the user a hint which registers had a problem,
>> even if they cannot really do anything about it right now.
>>
>> Sample output:
>>
>> Could not set register op0:3 op1:0 crn:0 crm:0 op2:0 to c00fac31 (is 413fd0c1)
>>
>> We could be even more helpful once we support writable ID registers,
>> at which point the user might actually be able to configure something
>> that is migratable.
>>
>> Suggested-by: Eric Auger <eric.auger@redhat.com>
>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>> ---
>>
>> Notes:
>> - This currently prints the list of failing registers for every call to
>>   write_list_to_kvmstate(), in particular for every cpu -- we might want
>>   to reduce that.
>> - If the macros aren't too ugly (or we manage to improve them), there
>>   might be other places where they could be useful.
>>
>> ---
>>  target/arm/kvm.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 53 insertions(+)
>>
>> diff --git a/target/arm/kvm.c b/target/arm/kvm.c
>> index 667234485547..ac6502e0c78f 100644
>> --- a/target/arm/kvm.c
>> +++ b/target/arm/kvm.c
>> @@ -900,6 +900,24 @@ bool write_kvmstate_to_list(ARMCPU *cpu)
>>      return ok;
>>  }
>>  
>> +/* pretty-print a KVM register */
>> +#define CP_REG_ARM64_SYSREG_OP(_reg, _op)                       \
>> +    ((uint8_t)((_reg & CP_REG_ARM64_SYSREG_ ## _op ## _MASK) >> \
>> +               CP_REG_ARM64_SYSREG_ ## _op ## _SHIFT))
>> +
>> +#define PRI_CP_REG_ARM64_SYSREG(_reg)                    \
>> +    ({                                                   \
>> +        char _out[32];                                   \
>> +        snprintf(_out, sizeof(_out),                     \
>> +                 "op0:%d op1:%d crn:%d crm:%d op2:%d",   \
>> +                 CP_REG_ARM64_SYSREG_OP(_reg, OP0),      \
>> +                 CP_REG_ARM64_SYSREG_OP(_reg, OP1),      \
>> +                 CP_REG_ARM64_SYSREG_OP(_reg, CRN),      \
>> +                 CP_REG_ARM64_SYSREG_OP(_reg, CRM),      \
>> +                 CP_REG_ARM64_SYSREG_OP(_reg, OP2));     \
>> +        _out;                                            \
>> +    })
>> +
> I am afraid this is too simplistic.
> Refering to linux/Documentation/virt/kvm/api.rst 4.68 KVM_SET_ONE_REG
> ARM registers section
> there are different groups of registers (upper 16b) and not all regs are
> further identified by op0-2, crn, crm.
> I think it would be valuable to output the group type and then the
> formatted lower 16b, depending on the group type.
>
> For instance 64b ARM FW pseudo reg is formatted as
> 0x6030 0000 0014 <regno:16>
>
> a diff on reg 0 results in
> qemu-system-aarch64: Could not set register op0:0 op1:0 crn:0 crm:0
> op2:0 to 10003 (is 10001)
> qemu-system-aarch64: error while loading state for instance 0x0 of
> device 'cpu'
> qemu-system-aarch64: Could not set register op0:0 op1:0 crn:0 crm:0
> op2:0 to 10003 (is 10001)
> qemu-system-aarch64: Failed to put registers after init: Invalid argument

You mean smth like

Could not set FW pseudo register <regno> to <val> (is <val>)
Could not set ID register <decoding> to <val> (is <val>)

?