Re: [PATCH v3 09/11] hw/arm/virt: Declare AArch32 DBGDTRTX as safe to ignore in incoming stream

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Nov 25 2025, 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 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>
>
> ---
>
> Theoretically removing that register could also impact other ARM
> machines but I don't think we have any compat outside of arm virt,
> do we?

I don't think we give any guarantees outside of versioned machines (of
which virt is the only one for ARM.)

> ---
>  hw/arm/virt.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index ec0af8e6e7..c6a5146c92 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -100,6 +100,14 @@ static GlobalProperty arm_virt_compat_defaults[] = {
>  static const size_t arm_virt_compat_defaults_len =
>      G_N_ELEMENTS(arm_virt_compat_defaults);
>  
> +#define DBGDTRTX 0x40200000200e0298

This probably wants a comment such as "bogus register erronously exposed
on 10.2 and earlier" or so.

> +
> +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);
> +
>  /*
>   * This cannot be called from the virt_machine_class_init() because
>   * TYPE_VIRT_MACHINE is abstract and mc->compat_props g_ptr_array_new()
> @@ -3536,6 +3544,7 @@ type_init(machvirt_machine_init);
>  
>  static void virt_machine_10_2_options(MachineClass *mc)
>  {
> +    compat_props_add(mc->compat_props, arm_virt_compat_10_2, arm_virt_compat_10_2_len);
>  }
>  DEFINE_VIRT_MACHINE_AS_LATEST(10, 2)
>  
> -- 
> 2.52.0