Re: [PATCH v3 1/5] [PATCH 1/5] gdb/aarch64: Add POR_EL0 register support for FEAT_S1POE

Luis <[email protected]> writes:

> On 14/07/2026 21:15, [email protected] wrote:
>> From: Srinath Parvathaneni <[email protected]>
>> Add support for the FEAT_S1POE POR_EL0 register on AArch64.
>> This patch adds POR_EL0 to the AArch64 register set and reads/writes it
>> using the NT_ARM_POE ptrace regset.
>
> Just a general comment, but por_el0 is unfortunate naming for a userspace register. But
> alas, it's been done that way in the Linux kernel as far as I can tell.

Do you mean the _el0 suffix? We had a discussion about it here:

https://inbox.sourceware.org/gdb-patches/[email protected]/

Srinath responded:

> That said, after discussing this with kernel/KVM developers, there are valid
> debugging scenarios (e.g. KGDB or guest debugging via KVM/QEMU) where exposing
> `POR_EL0`, `POR_EL1`, and `POR_EL2` simultaneously would be useful. This seems
> like a broader GDB register naming issue rather than something specific to
> FEAT_S1POE.

And Marc Zyngier too:

> It would certainly make our life easier if GDB was in general adopting
> the architecture nomenclature.
> 
> It is probably fine to have a "shorthand" such as POR for POR_EL0, but
> I'd like to make sure that it is possible to unambiguously target the
> correct register for the cases where we have to debug a full guest
> (which is something people actively do using the QEMU GDB stubs).

So as Marc says perhaps we could have an alias por for por_el0, like we
have lr for x30, or (in the other direction) x31 for sp?

>> diff --git a/gdb/features/aarch64-poe.c b/gdb/features/aarch64-poe.c
>> new file mode 100644
>> index 00000000000..4bd795e9fe6
>> --- /dev/null
>> +++ b/gdb/features/aarch64-poe.c
>> @@ -0,0 +1,68 @@
>> +/* THIS FILE IS GENERATED.  -*- buffer-read-only: t -*- vi:set ro:
>> +  Original: aarch64-poe.xml */
>> +
>> +#include "gdbsupport/tdesc.h"
>> +
>> +static int
>> +create_feature_aarch64_poe (struct target_desc *result, long regnum)
>> +{
>> +  struct tdesc_feature *feature;
>> +
>> +  feature = tdesc_create_feature (result, "org.gnu.gdb.aarch64.poe");
>> +  tdesc_type_with_fields *type_with_fields;
>> +  type_with_fields = tdesc_create_enum (feature, "por_el0_fmt", 4);
>> +  tdesc_add_enum_value (type_with_fields, 0, "---");
>> +  tdesc_add_enum_value (type_with_fields, 1, "r--");
>> +  tdesc_add_enum_value (type_with_fields, 2, "--x");
>> +  tdesc_add_enum_value (type_with_fields, 3, "r-x");
>> +  tdesc_add_enum_value (type_with_fields, 4, "-w-");
>> +  tdesc_add_enum_value (type_with_fields, 5, "rw-");
>> +  tdesc_add_enum_value (type_with_fields, 6, "-wx");
>> +  tdesc_add_enum_value (type_with_fields, 7, "rwx");
>> +  tdesc_add_enum_value (type_with_fields, 8, "???");
>> +  tdesc_add_enum_value (type_with_fields, 9, "???");
>> +  tdesc_add_enum_value (type_with_fields, 10, "???");
>> +  tdesc_add_enum_value (type_with_fields, 11, "???");
>> +  tdesc_add_enum_value (type_with_fields, 12, "???");
>> +  tdesc_add_enum_value (type_with_fields, 13, "???");
>> +  tdesc_add_enum_value (type_with_fields, 14, "???");
>> +  tdesc_add_enum_value (type_with_fields, 15, "???");
>
> I'm not a fan of this. Have we considered alternatives like pseudo-registers that map
> to/from the raw POR value?
>
> It feels like this is working around a gdb deficiency of not having a proper type, and the
> right way to solve this would be extending gdb in some way.
>
> With a pseudo-register all of this would be interiorized in gdb, and we would be left with
> only the raw POR value of 64 bits.
>
>> +
>> +  type_with_fields = tdesc_create_flags (feature, "por_el0_flags", 8);
>> +  tdesc_type *field_type;
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P15", 60, 63, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P14", 56, 59, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P13", 52, 55, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P12", 48, 51, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P11", 44, 47, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P10", 40, 43, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P9", 36, 39, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P8", 32, 35, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P7", 28, 31, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P6", 24, 27, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P5", 20, 23, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P4", 16, 19, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P3", 12, 15, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P2", 8, 11, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P1", 4, 7, field_type);
>> +  field_type = tdesc_named_type (feature, "por_el0_fmt");
>> +  tdesc_add_typed_bitfield (type_with_fields, "P0", 0, 3, field_type);
>
> Likewise for the above. This is hardcoding the interpretation of the individual bitfields
> into the XML.
>
> Maybe Thiago has a different opinion here.

The interpretation is fixed by the architecture, so hardcoding it makes
sense IMHO. But I don't feel strongly about this.

-- 
Thiago
(he/him)