Re: [PATCH v6 08/11] hw/arm/virt: Declare AArch32 DBGDTRTX as safe to ignore in incoming stream

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jan 26 2026, Eric Auger <eric.auger@redhat.com> wrote:

> With the new infrastructure in place it is now feasible to teach
> qemu that it is safe to ignore a sysreg in the incoming migration
> stream. So with the plan to revert commit 4f2b82f60431 ("target/arm:
> Reinstate bogus AArch32 DBGDTRTX register for migration compat") from
> qemu 11.0 onwards, let's add a compat in 10.2 machine options stating
> that this reg is safe to ignore. from 11.0 onwards we will not need
> that register anymore.
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
>
> ---
>
> v4 -> v5:
> - rebased on top of latest machine types (Connie)
>
> v3 -> v4:
> - add a comment related to DBGDTRTX (Connie)
> ---
>  hw/arm/virt.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index baa4e31aac1..03d5af18f26 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -100,6 +100,15 @@ static GlobalProperty arm_virt_compat_defaults[] = {
>  static const size_t arm_virt_compat_defaults_len =
>      G_N_ELEMENTS(arm_virt_compat_defaults);
>  
> +/* Register erronously exposed on 10.2 and earlier */
> +#define DBGDTRTX 0x40200000200e0298
> +
> +static GlobalProperty arm_virt_compat_10_2[] = {
> +    { TYPE_ARM_CPU, "x-mig-safe-missing-regs", stringify(DBGDTRTX)},
> +};
> +static const size_t arm_virt_compat_10_2_len =
> +    G_N_ELEMENTS(arm_virt_compat_10_2);
> +

Not objecting, but we had a discussion recently regarding where compat
values for arm cpus should live:
https://lore.kernel.org/qemu-devel/20260120122108.131708-1-thuth@redhat.com/ ff.

Could this become relevant for future other versioned machine types? I'd
assume that they just would skip the bogus reg from the start, though.

>  /*
>   * This cannot be called from the virt_machine_class_init() because
>   * TYPE_VIRT_MACHINE is abstract and mc->compat_props g_ptr_array_new()
> @@ -3552,6 +3561,7 @@ static void virt_machine_10_2_options(MachineClass *mc)
>  {
>      virt_machine_11_0_options(mc);
>      compat_props_add(mc->compat_props, hw_compat_10_2, hw_compat_10_2_len);
> +    compat_props_add(mc->compat_props, arm_virt_compat_10_2, arm_virt_compat_10_2_len);
>  }
>  DEFINE_VIRT_MACHINE(10, 2)
>