gdb XML type for SME ZA storage

[Luis]

On 13/09/2025 20:02, Luis wrote:
> The documentation is probably off here, as the intention was to have a 
> 2-dimensional array of bytes.
> 
> Let me confirm that and then I can fix the documentation.
> 

So ZA is really a 2d array of bytes, so a vector of vector of bytes.

Here's the XML gdb outputs for sme and sme2:

   <feature name="org.gnu.gdb.aarch64.sme">
     <flags id="svcr_flags" size="8">
       <field name="SM" start="0" end="0" type="bool"/>
       <field name="ZA" start="1" end="1" type="bool"/>
     </flags>
     <vector id="sme_bv" type="uint8" count="32"/>
     <vector id="sme_bvv" type="sme_bv" count="32"/>
     <reg name="svg" bitsize="64" type="int" regnum="91"/>
     <reg name="svcr" bitsize="64" type="svcr_flags" regnum="92"/>
     <reg name="za" bitsize="8192" type="sme_bvv" regnum="93"/>
   </feature>
   <feature name="org.gnu.gdb.aarch64.sme2">
     <vector id="sme2_bv" type="uint8" count="64"/>
     <reg name="zt0" bitsize="512" type="sme2_bv" regnum="94"/>
   </feature>

> On Thu, Sep 11, 2025, 18:28 Peter Maydell <peter.maydell@linaro.org 
> <mailto:peter.maydell@linaro.org>> wrote:
> 
>     Hi; in implementing the QEMU gdbstub handling for SME registers,
>     we're not sure about the right type for the ZA storage. The docs say
> 
>     https://sourceware.org/gdb/current/onlinedocs/gdb.html/AArch64-
>     Features.html#AArch64-SME-registers-feature <https://sourceware.org/
>     gdb/current/onlinedocs/gdb.html/AArch64-Features.html#AArch64-SME-
>     registers-feature>
> 
>     "- ZA is a register represented by a vector of svlxsvl bytes."
> 
>     but the code does this:
> 
>        /* Byte type.  */
>        element_type = tdesc_named_type (feature, "uint8");
>        /* Vector of bytes.  */
>        element_type = tdesc_create_vector (feature, "sme_bv", element_type,
>                                            svl);
>        /* Vector of vector of bytes (Matrix).  */
>        element_type = tdesc_create_vector (feature, "sme_bvv", element_type,
>                                            svl);
> 
>        /* The following is the ZA register set.  */
>        tdesc_create_reg (feature, "za", regnum++, 1, nullptr,
>                          std::pow (svl, 2) * 8, "sme_bvv");
> 
>     which looks like it sets up the XML as a vector of a vector of bytes,
>     rather than a simple vector of bytes. (I did want to try to run
>     gdbserver to dump the actual XML it outputs, but wasn't able to
>     get a working setup for that.)
> 
>     Are the docs wrong, or the code, or are "vector of N*N bytes" and
>     "vector of N vectors of N bytes" the same type as far as GDB is
>     concerned, such that it doesn't matter which one the stub reports
>     in the XML ?
> 
>     thanks
>     -- PMM
>