gdb XML type for SME ZA storage

[Peter Maydell]

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

"- 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