Re: gdb support for SME-without-SVE ?

[Simon Marchi via Gdb <gdb@sourceware.org>]

On 2026-01-27 06:31, Peter Maydell via Gdb wrote:
> Hi: is GDB for Arm intended to support configurations where the target
> CPU has SME but not SVE?
> 
> We're just implementing support for that in QEMU for using SME
> with the hvf hypervisor accelerator on macos systems, but when
> we tried connecting gdb to QEMU gdb crashed:
> 
>   (gdb) target remote localhost:1234
>   Remote debugging using localhost:1234
>   ../../gdb/aarch64-tdep.c:3068: internal-error:
> aarch64_pseudo_register_type: bad register number 160
>   A problem internal to GDB has been detected,
>   further debugging may prove unreliable.
>   Fatal signal: Abort trap: 6
> 
> https://lore.kernel.org/qemu-devel/CAAjaMXZLG2aBtStRhyvmdENj1Z+Mx05BmDgyYUoYrc_ZnHwyVQ@mail.gmail.com/
> 
> Is this a known missing feature in GDB, or is it a config that's
> supposed to work but we've got the XML register description wrong
> somehow?
> 
> It looks from the aarch64-tdep.c code like perhaps GDB assumes
> that the Vn vector registers only exist with SVE, but they also
> exist in SME-only CPUs.

Not an ARM expert, but from what I read, SME builds on top of SVE, so I
would guess that having SME implies having SVE.  For instance, from [1]:

  Arm Scalable Matrix Extension (SME) is an architecture extension that
  provides enhanced support for matrix operations. SME builds on the
  Scalable Vector Extensions (SVE and SVE2), adding new capabilities to
  efficiently process matrices. Key features include...

Simon

[1] https://developer.arm.com/community/arm-community-blogs/b/architectures-and-processors-blog/posts/arm-scalable-matrix-extension-introduction