Re: gdb support for SME-without-SVE ?

[Thiago Jung Bauermann via Gdb <gdb@sourceware.org>]

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.

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.

-- 
Thiago