[PATCH,v3 17/17] [gdb/docs] sme: Document SME registers and features

[Luis Machado <luis.machado@arm.com>]

Updates since v2:

- More adjustments based on reviews.
- Fixed incorrect number of tile pseudo-registers.
- Fixed naming of tile slice pseudo-registers.
- More detail about SME and how gdb implements it.
- Attempted to clarify the text a bit more.

Updates since v1:

- Made SME text more thorough.
- Adjusted text based on upstream reviews.
- Fixed documentation errors (missing itemization for SME registers).

Provide documentation for the SME feature and other information that
should be useful for users that need to debug a SME-capable target.
---
 gdb/NEWS            |  11 ++
 gdb/doc/gdb.texinfo | 249 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 260 insertions(+)

diff --git a/gdb/NEWS b/gdb/NEWS
index 54b5da21245..e3ce7d7e881 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -3,6 +3,17 @@
 
 *** Changes since GDB 13
 
+* GDB now supports the AArch64 Scalable Matrix Extension (SME), which includes
+  a new matrix register named ZA, a new thread register TPIDR2 and a new vector
+  length register SVG (streaming vector granule).  GDB also supports tracking
+  ZA state across signal frames.
+
+  Some features are still under development or are dependent on ABI specs that
+  are still in alpha stage.  For example, manual function calls with ZA state
+  don't have any special handling, and tracking of SVG changes based on
+  DWARF information is still not implemented, but there are plans to do so in
+  the future.
+
 * The AArch64 'org.gnu.gdb.aarch64.pauth' Pointer Authentication feature string
   has been deprecated in favor of the 'org.gnu.gdb.aarch64.pauth_v2' feature
   string.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 940315b2713..3149e5a6765 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -26045,6 +26045,224 @@ but the lengths of the @code{z} and @code{p} registers will not change.  This
 is a known limitation of @value{GDBN} and does not affect the execution of the
 target process.
 
+For SVE, the following definitions are used throughout @value{GDBN}'s source
+code and in this document:
+
+@itemize
+
+@item
+@anchor{VL}
+@cindex VL
+@code{VL}: The vector length, in bytes.  It defines the size of each @code{Z}
+register.
+
+@item
+@anchor{VQ}
+@cindex VQ
+@code{VQ}: The number of 128 bit units in @code{VL}.  This is mostly used
+internally by @value{GDBN} and the Linux Kernel.
+
+@item
+@anchor{VG}
+@cindex VG
+@code{VG}: The number of 64 bit units in @code{VL}.  This is mostly used
+internally by @value{GDBN} and the Linux Kernel.
+
+@end itemize
+
+@subsubsection AArch64 SME.
+@anchor{AArch64 SME}
+@cindex SME
+@cindex AArch64 SME
+@cindex Scalable Matrix Extension
+
+The Scalable Matrix Extension (@url{https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/scalable-matrix-extension-armv9-a-architecture, @acronym{SME}})
+is an AArch64 architecture extension that expands on the concept of the
+Scalable Vector Extension (@url{https://developer.arm.com/documentation/101726/4-0/Learn-about-the-Scalable-Vector-Extension--SVE-/What-is-the-Scalable-Vector-Extension-, @acronym{SVE}})
+by providing a 2-dimensional register @code{ZA}, which is a square
+matrix of variable size, just like SVE provides a group of vector registers of
+variable size.
+
+Similarly to SVE, where the size of each @code{Z} register is directly related
+to the vector length (@code{VL} for short), the @acronym{SME} @code{ZA} matrix
+register's size is directly related to the streaming vector length
+(@code{SVL} for short). @xref{VL} @xref{SVL}
+
+The @code{ZA} matrix state can be either active or inactive, if it is not in
+use.
+
+@acronym{SME} also introduces a new execution mode called streaming
+@acronym{SVE} mode (streaming mode for short).  When streaming mode is
+enabled, the program supports execution of @acronym{SVE2} instructions and the
+@acronym{SVE} registers will have vector length @code{SVL}.  When streaming
+mode is disabled, the SVE registers have vector length @code{VL}.
+
+For more information about @acronym{SME} and @acronym{SVE}, please refer to
+official @url{https://developer.arm.com/documentation/ddi0487/latest,
+architecture documentation}.
+
+The following definitions are used throughout @value{GDBN}'s source code and
+in this document:
+
+@itemize
+
+@item
+@anchor{SVL}
+@cindex SVL
+@code{SVL}: The streaming vector length, in bytes.  It defines the size of each
+dimension of the 2-dimensional square @code{ZA} matrix.  The total size of
+@code{ZA} is therefore @code{@var{SVL}x@var{SVL}}.
+
+When streaming mode is enabled, it defines the size of the @acronym{SVE}
+registers as well.
+
+@item
+@anchor{SVQ}
+@cindex SVQ
+@code{SVQ}: The number of 128 bit units in @code{SVL}.  This is mostly used
+internally by @value{GDBN} and the Linux Kernel.
+
+@item
+@anchor{SVG}
+@cindex SVG
+@code{SVG}: The number of 64 bit units in @code{SVL}.  This is mostly used
+internally by @value{GDBN} and the Linux Kernel.
+
+@end itemize
+
+When @value{GDBN} is debugging the AArch64 architecture, if the Scalable Matrix
+Extension (@acronym{SME}) is present, then @value{GDBN} will make the @code{za}
+register available.  @value{GDBN} will also make the @code{svg} register and
+@code{svcr} pseudo-register available.
+
+The @code{za} register is a 2-dimensional square @code{@var{SVL}x@var{SVL}}
+matrix of bytes.  To simplify the representation and access to the @code{za}
+register in @value{GDBN}, it is defined as a vector of
+@code{@var{SVL}x@var{SVL}} bytes.
+
+If the user wants to index the @code{za} register as a matrix, it is possible
+to reference @code{za} as @code{za[@var{i}][@var{j}]}, where @var{i} is the
+row number and @var{j} is the column number.
+
+The @code{svg} register always contains the streaming vector granule
+(@code{SVG}) for the current thread.  From @code{SVG} we can easily derive
+the @code{SVL} value.
+
+@anchor{aarch64 sme svcr}
+The @code{svcr} pseudo-register (streaming vector control register) is a status
+register that holds two state bits: @sc{sm} in bit 0 and @sc{za} in bit 1.
+
+If the @sc{sm} bit is 1, it means the current thread is in streaming
+mode, and the @acronym{SVE} registers will use @code{SVL} for their sizes.  If
+the @sc{sm} bit is 0, the current thread is not in streaming mode, and the
+@acronym{SVE} registers will use @code{VG} for their sizes.  @xref{VG}.
+
+If the @sc{za} bit is 1, it means the @code{ZA} state, and therefore the
+@code{za} register, is being used and has meaningful contents.  If the
+@sc{za} bit is 0, the @code{ZA} state is unavailable and the contents of the
+@code{za} register are undefined.
+
+For convenience and simplicity, if the @sc{za} bit is 0, the @code{za}
+register and all of its pseudo-registers will read as zero.
+
+If @code{SVL} changes during the execution of a program, then the @code{za}
+register size and the bits in the @code{svcr} pseudo-register will be updated
+to reflect it.
+
+It is possible for users to change @code{SVL} during the execution of a
+program by modifying the @code{svg} register value.
+
+Whenever the @code{svg} register is modified with a new value, the
+following will be observed:
+
+@itemize
+
+@item The @sc{za} and @sc{sm} bits will be cleared in the @code{svcr}
+pseudo-register.
+
+@item The @code{za} register will have a new size and its state will be
+cleared, forcing its contents and the contents of all of its pseudo-registers
+back to zero.
+
+@item If the @sc{sm} bit was 1, the @acronym{SVE} registers will be reset to
+having their sizes based on @code{VL} as opposed to @code{SVL}.  If the
+@sc{sm} bit was 0 prior to modifying the @code{svg} register, there will be no
+observable effect on the @acronym{SVE} registers.
+
+@end itemize
+
+The possible values for the @code{svg} register are 2, 4, 8, 16, 32.  These
+numbers correspond to streaming vector length (@code{SVL}) values of 16
+bytes, 32 bytes, 64 bytes, 128 bytes and 256 bytes respectively.
+
+The minimum size of the @code{za} register is 16 x 16 (256) bytes, and the
+maximum size is 256 x 256 (65536) bytes.  In streaming mode, with bit @sc{sm}
+set, the size of the @code{za} register is the size of all the SVE @code{z}
+registers combined.
+
+The @code{za} register can also be accessed using tiles and tile slices.
+
+Tile pseudo-registers are square, 2-dimensional sub-arrays of elements within
+the @code{za} register.
+
+There is a total of 31 @code{za} tile pseudo-registers.  They are
+@code{za0b}, @code{za0h} through @code{za1h}, @code{zas0} through @code{zas3},
+@code{zad0} through @code{zad7} and @code{zaq0} through @code{zaq15}.
+
+Tile slice pseudo-registers are vectors of horizontally or vertically
+contiguous elements within the @code{za} register.
+
+The tile slice pseudo-registers have the following naming pattern:
+@code{za<@var{tile number}><@var{direction}><@var{qualifier}>
+<@var{slice number}>}.
+
+There are up to 16 tiles (0 ~ 15), the direction can be either @code{v}
+(vertical) or @code{h} (horizontal), the qualifiers can be @code{b} (byte),
+@code{h} (halfword), @code{s} (word), @code{d} (doubleword) and @code{q}
+(quadword) and there are up to 256 slices (0 ~ 255) depending on the value
+of @code{SVL}.  The number of slices is the same as the value of @code{SVL}.
+
+The number of available tile slice pseudo-registers can be large.  For a
+minimum @code{SVL} of 16 bytes, there are 5 (number of qualifiers) x
+2 (number of directions) x 16 (@code{SVL}) pseudo-registers.  For the
+maximum @code{SVL} of 256 bytes, there are 5 x 2 x 256 pseudo-registers.
+
+When listing all the available registers, users will see the
+currently-available @code{za} pseudo-registers.  Pseudo-registers that don't
+exist for a given @code{SVL} value will not be displayed.
+
+For more information on @acronym{SME} and its terminology, please refer to the
+@url{https://developer.arm.com/documentation/ddi0616/aa/,
+Arm Architecture Reference Manual Supplement}, The Scalable Matrix Extension
+(@acronym{SME}), for Armv9-A.
+
+Some features are still under development and rely on
+@url{https://github.com/ARM-software/acle/releases/latest, ACLE} and
+@url{https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst, ABI}
+definitions, so there are known limitations to the current @acronym{SME}
+support in @value{GDBN}.
+
+One such example is calling functions in the program being debugged by
+@value{GDBN}.  Such calls are not @acronym{SME}-aware and thus don't take into
+account the @code{svcr} pseudo-register bits nor the @code{za} register
+contents. @xref{Calling}
+
+The @url{https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#the-za-lazy-saving-scheme,
+lazy saving scheme} involving the @code{tpidr2} register is not yet supported
+by @value{GDBN}, though the @code{tpidr2} register is known and supported
+by @value{GDBN}.
+
+Lastly, an important limitation for @code{gdbserver} is its inability to
+communicate @code{SVL} changes to @value{GDBN}.  This means @code{gdbserver},
+even though it is capable of adjusting its internal caches to reflect a change
+in the value of @code{SVL} mid-execution, will operate with a potentially
+different @code{SVL} value compared to @value{GDBN}.  This can lead to
+@value{GDBN} showing incorrect values for the @code{za} register and
+incorrect values for SVE registers (when in streaming mode).
+
+This is the same limitation we have for the @acronym{SVE} registers, and there
+are plans to address this limitation going forward.
+
 @subsubsection AArch64 Pointer Authentication.
 @cindex AArch64 Pointer Authentication.
 @anchor{AArch64 PAC}
@@ -48071,6 +48289,37 @@ This restriction may be lifted in the future.
 Extra registers are allowed in this feature, but they will not affect
 @value{GDBN}.
 
+@subsubsection AArch64 SME registers feature
+
+The @samp{org.gnu.gdb.aarch64.sme} feature is optional.  If present,
+it should contain registers @samp{za}, @samp{svg} and @samp{svcr}.
+@xref{AArch64 SME}
+
+@itemize @minus
+
+@item
+@samp{za} is a register represented by a vector of @code{SVL} x @code{SVL}
+bytes.  @xref{SVL}
+
+@item
+@samp{svg} is a 64-bit register containing the value of @code{SVG}.  @xref{SVG}.
+
+@item
+@samp{svcr} is a 64-bit status pseudo-register with two valid bits.  Bit 0
+(@sc{sm}) shows whether the streaming @acronym{SVE} mode is enabled or disabled.
+Bit 1 (@sc{za}) shows whether the @code{za} register state is active (in use) or
+not.
+@xref{aarch64 sme svcr}
+
+The rest of the unused bits of the @samp{svcr} pseudo-register is undefined
+and reserved.  Such bits should not be used and may be defined by future
+extensions of the architecture.
+
+@end itemize
+
+Extra registers are allowed in this feature, but they will not affect
+@value{GDBN}.
+
 @node ARC Features
 @subsection ARC Features
 @cindex target descriptions, ARC Features
-- 
2.25.1