Re: [PATCH v2 2/8] target/arm/cpu: Allow registers to be hidden

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

Hi Connie,

On 11/19/25 5:35 PM, Cornelia Huck wrote:
> On Tue, Nov 18 2025, Eric Auger <eric.auger@redhat.com> wrote:
>
>> More recent kernels sometimes expose new registers in an
>> unconditionnal manner. This situation breaks backward migration
>> as qemu notices there are more registers in the input stream
>> than supported on the destination host. This leads to a
>> "failed to load cpu:cpreg_vmstate_array_len" error.
>>
>> A good example is the introduction of KVM_REG_ARM_VENDOR_HYP_BMAP_2
>> pseudo FW register in v6.16 by commit C0000e58c74e (“KVM: arm64:
>> Introduce KVM_REG_ARM_VENDOR_HYP_BMAP_2”). Trying to do backward
>> migration from a host kernel that features the commit to a destination
>> host that doesn't, fail with above error.
>>
>> Currently QEMU is not using that feature so ignoring this latter
>> is not a problem. An easy way to fix the migration issue is to teach
>> qemu we don't care about that register and we can simply ignore it
>> when syncing its state during migration.
>>
>> This patch introduces an array of such hidden registers. Soon it will
>> be settable through an array property.
>>
>> If hidden, the register is moved out of the array of cpreg which is
>> built in kvm_arm_init_cpreg_list(). That way their state won't be
>> synced.
>>
>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>>
>> ---
>>
>> v1 -> v2:
>> - Move the property in a separate patch
>> - improve the commit msg
>> - change the trace point to just print info in
>>   kvm_arm_init_cpreg_list()
>> - improve comment in cpu.h (Connie)
>> ---
>>  target/arm/cpu.h        | 23 +++++++++++++++++++++++
>>  target/arm/kvm.c        | 12 +++++++++++-
>>  target/arm/trace-events |  2 ++
>>  3 files changed, 36 insertions(+), 1 deletion(-)
>>
>> diff --git a/target/arm/cpu.h b/target/arm/cpu.h
>> index 077b0cce5b..0a283940be 100644
>> --- a/target/arm/cpu.h
>> +++ b/target/arm/cpu.h
>> @@ -1044,6 +1044,18 @@ struct ArchCPU {
>>      /* KVM steal time */
>>      OnOffAuto kvm_steal_time;
>>  
>> +    /*
>> +     * Register indexes that must be hidden. Although normally
>> +     * supported (defined in TCG description or exposed by KVM) they are
>> +     * willingly hidden for migration sake. This may be used to allow
>> +     * backward migration to older versions that do implement a specific
>> +     * feature. With KVM acceleration the indexes are the ones described
>> +     * in linux/Documentation/virt/kvm/api.rst. With TCG, this is the TCG
>> +     * sysreg index.
>> +     */
> Hmm... what about
>
> "Array of register indexes that need to be hidden to allow migration in
> certain cases, i.e. when a register is exposed in KVM or defined in TCG
> but not actually used in QEMU. For the KVM case, the indexes are as
> described in Linux Documentation/virt/kvm/api.rst. For TCG, the indexes
> are the TCG sysreg indexes."
sounds good.

about the TCG index, I am not sure. It rather looks 
cpreg_to_kvm_id(ENCODE_CP_REG()) in the case of Aarch32 DBGDTRTX index I
need to use to succeed the migration



>
>> +    uint64_t *hidden_regs;
>> +    uint32_t nr_hidden_regs;
>> +
>>      /* Uniprocessor system with MP extensions */
>>      bool mp_is_up;
>>  
> Otherwise, LGTM.

Thanks!

Eric
>