Re: gdb support for SME-without-SVE ?

[Luis via Gdb <gdb@sourceware.org>]

On Wed, Jan 28, 2026, 04:24 Thiago Jung Bauermann <
thiago.bauermann@linaro.org> wrote:

> Hello,
>
> I'm currently implementing SME-only (i.e. without SVE) support in GDB
> for Linux userspace debugging. It's mostly done, I just need to test a
> few things, polish the code and prepare the patches. I'm hoping to send
> the patches next week.
>

Excellent. Thanks for the heads up.



> Due to the limitations in the remote protocol that Luis mentioned below,
> I can't run the tests against a remote target so I'm only focusing on
> local debugging, unfortunately.
>
> Luis <luis.machado.foss@gmail.com> writes:
>
> > On Tue, Jan 27, 2026, 21:24 Peter Maydell <peter.maydell@linaro.org>
> wrote:
> >
> > > (2) If the CPU has SME and not SVE, presumably we should still
> > > expose org.gnu.gdb.aarch64.sve, as it's where the z registers are.
> > > What should we report "vl" as here? How about when the CPU is not
> > > in streaming mode and the z regs aren't accessible?
> >
> > Are there still Z registers without SVE?
>
> Yes, but they are only accessible while in streaming mode.
>
> For simplicity, if the inferior isn't in streaming mode I'm making the
> SVE registers available in non-streaming mode (i.e., behave as if SVE is
> supported) with vg = 2 as Peter mentions below. In this case, the Z
> registers are equivalent to the regular vector registers.
>
> > I don't think this is properly supported in any case. On top of that
> > SVE and SME are not properly supported via the remote protocol.  In
> > theory it works if you don't change vg/svg mid-execution. So no SSVE
> > enabling etc. But in practice it does not work well. When the vg/svg
> > size changes you end up with overruns or garbage.
> >
> > The initial implementation of SVE assumed remote debugging wasn't
> > important. We're stuck with that until remote protocol enhancements
> > are contributed.
> >
> > Those enhancements have been proposed multiple times but there were
> > always downsides and it tends to die and fall into limbo.  It mostly
> > only affects SVE and SME. Maybe AMX.
>
> I'm planning to resume this work again in the next few months. Christina
> and Klaus from Intel came up with a nice proposal.
>

Great. Hopefully we can get this restriction lifted and enable dynamic
sizes for remote stubs.



> --
> Thiago
>