On Tue, 25 Nov 2025, Eric Auger 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 <[email protected]>
Reviewed-by: Sebastian Ott <[email protected]>
