On 1/27/26 6:09 PM, Cornelia Huck wrote:
> On Mon, Jan 26 2026, Eric Auger <[email protected]> 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.
>>
>> To extend that functionality to TCG, do the same in add_cpreg_to_list()
>> and count_cpreg().
>>
>> Signed-off-by: Eric Auger <[email protected]>
>> Reviewed-by: Sebastian Ott <[email protected]>
>>
>> ---
>> v2 -> v3:
>> - use kvm_regidx
>>
>> 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/helper: Skip hidden registers
>>
>> In case a cpreg is hidden, skip it when initialing the cpreg
>> list.
>>
>> Signed-off-by: Eric Auger <[email protected]>
>> Reviewed-by: Cornelia Huck <[email protected]>
> This looks a bit weird, probably due to patch folding... but I'm happy
> to have my R-b apply to the whole patch :)
Oh yes I missed that. Nevertheless it is below the --- so I guess it is
not that much an issue
Eric
>
>> ---
>> target/arm/cpu.h | 20 ++++++++++++++++++++
>> target/arm/helper.c | 12 +++++++++++-
>> target/arm/kvm.c | 12 +++++++++++-
>> target/arm/trace-events | 2 ++
>> 4 files changed, 44 insertions(+), 2 deletions(-)
