Re: [PATCH v4 03/10] target/arm/cpu: Allow registers to be hidden

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Dec 22 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.
>
> To extend that functionality to TCG, do the same in add_cpreg_to_list()
> and count_cpreg().
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> Reviewed-by: Sebastian Ott <sebott@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/helper: Skip hidden registers
>
> In case a cpreg is hidden, skip it when initialing the cpreg
> list.
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>
> ---
>
> v2 -> v3:
> - use kvm_regidx

This looks a bit odd after squashing the patches, maybe edit the patch
description?

Otherwise,

Reviewed-by: Cornelia Huck <cohuck@redhat.com>

> ---
>  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(-)