Re: [PATCH] docs: Clarify gdb remote protocol AArch64 SVE and SME handling

[Luis <luis.machado.foss@gmail.com>]

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On Thu, Jan 29, 2026, 18:11 Peter Maydell <peter.maydell@linaro.org> wrote:

> The documentation of the AArch64 target features for the gdb remote
> protocol has some areas where it is unclear about SVE and SME:
>
>  * it doesn't say what to do if the target has only SME
>
>  * it isn't clear about what the org.gnu.gdb.aarch64.sve vector
>    register size should be when both SME and SVE are present
>
> Clarify/correct the documentation:
>
>  * org.gnu.gdb.aarch64.sve is effectively "the feature for the z
>    vector registers", and must be provided even when the target only
>    implements SME (because the z registers exist in an SME only CPU)
>
>  * the z register size and the 'vg' pseudoregister in
>    org.gnu.gdb.aarch64.sve follow the architectural "effective SVE
>    vector length", which might be either the non-streaming SVE vector
>    length or the streaming SVE vector length
>
> Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
> ---
>  gdb/doc/gdb.texinfo | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index 80f49e21b7e..711a45435ef 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -49742,7 +49742,8 @@ Extra registers are allowed in this feature, but
> they will not affect
>  @subsubsection AArch64 SVE registers feature
>
>  The @samp{org.gnu.gdb.aarch64.sve} feature is optional.  If present,
> -it means the target supports the Scalable Vector Extension and must
> contain
> +it means the target supports either the Scalable Vector Extension (SVE) or
> +the Scalable Matrix Extension (SME) and must contain
>  the following registers:
>
>  @itemize @minus
> @@ -49780,6 +49781,14 @@ vector registers from the
> @samp{org.gnu.gdb.aarch64.fpu} feature.
>  The first 128 bits of the @samp{z} registers overlap the 128 bits of the
>  @samp{v} registers, so changing one will trigger a change to the other.
>
> +@samp{vg} represents the size of the @samp{z} registers, and is what
> +the Arm architecture defines as the ``Effective SVE vector length''. That
> +means that if the target implements SME and is in streaming vector mode,
> +it is the streaming vector length. If the target implements only SME and
> +not SVE, and is not in streaming vector mode, then @samp{vg} is 2
> +(indicating 128-bit vectors) and the @samp{z} registers match the @samp{v}
> +FPU registers.
> +
>  For the types of the @samp{z}, @samp{p} and @samp{ffr} registers, please
>  check the aarch64-sve.c file.  No XML file is available for this feature
>  because it is dynamically generated based on the current vector length.
> @@ -49793,6 +49802,9 @@ aarch64-sve.c file, and should match what is
> described in aarch64-fpu.xml.
>  Extra registers are allowed in this feature, but they will not affect
>  @value{GDBN}.
>
> +The @samp{org.gnu.gdb.aarch64.sve} feature is required when the target
> also
> +reports support for the @samp{org.gnu.gdb.aarch64.sme} feature.
> +
>  @subsubsection AArch64 Pointer Authentication registers feature
>
>  The @samp{org.gnu.gdb.aarch64.pauth} optional feature was introduced so
> @@ -49948,6 +49960,10 @@ extensions of the architecture.
>  Extra registers are allowed in this feature, but they will not affect
>  @value{GDBN}.
>
> +Note that a target with only SME but not SVE support must still
> +provide the @samp{org.gnu.gdb.aarch64.sve} feature, to expose the
> +@samp{z} vector registers.
> +
>  The @samp{org.gnu.gdb.aarch64.sme} feature is required when the target
> also
>  reports support for the @samp{org.gnu.gdb.aarch64.sme2} feature.
>
> --
> 2.43.0
>

Thanks. The documentation update looks OK given the current needs, but I'm
not too sure about keeping the SVE feature just for the Z registers. I find
that a bit confusing.

It might be cleaner to clearly separate SVE and SME features, adding Z
registers to a SME feature if needed when SVE is disabled. Or something
similar to that.

I don't feel comfortable setting this is stone before we have a patch that
clarifies how this separation between SVE and SME is achieved.

>

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