Re: [PATCH v5 1/4] target/arm: Increase MAX_PACKET_LENGTH for SME ZA

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 11 Aug 2025 at 20:37, Vacha Bhavsar
<vacha.bhavsar@oss.qualcomm.com> wrote:
>
> This patch increases the value of the MAX_PACKET_LEGNTH to
> 131104 from 4096 to allow the GDBState.line_buf to be large enough
> to accommodate the full contents of the SME ZA storage when the
> vector length is maximal. This is in preparation for a related
> patch that allows SME register visibility through remote GDB
> debugging.
>
> Signed-off-by: Vacha Bhavsar <vacha.bhavsar@oss.qualcomm.com>
> ---
> Changes since v4:
> - the value for MAX_PACKET_LENGTH is changed from 131100 to
> 131104 to align with a similar update made to gdbserver
> ---
>  gdbstub/internals.h | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/gdbstub/internals.h b/gdbstub/internals.h
> index bf5a5c6302..87f64b6318 100644
> --- a/gdbstub/internals.h
> +++ b/gdbstub/internals.h
> @@ -11,7 +11,27 @@
>
>  #include "exec/cpu-common.h"
>
> -#define MAX_PACKET_LENGTH 4096
> +/*
> +* Most "large" transfers (e.g. memory reads, feature XML
> +* transfer) have mechanisms in the gdb protocol for splitting
> +* them. However, register values in particular cannot currently
> +* be split. This packet size must therefore be at least big enough
> +* for the worst-case register size. Currently that is Arm SME
> +* ZA storage with a 256x256 byte value. We also must account
> +* for the conversion from raw data to hex in gdb_memtohex(),
> +* which writes 2 * size bytes, and for other protocol overhead
> +* including command, register number and checksum which add
> +* another 4 bytes of overhead. However, to be consistent with
> +* the changes made in gdbserver to address this same requirement,
> +* we add a total of 32 bytes to account for protocol overhead
> +* (unclear why specifically 32 bytes), bringing the value of
> +* MAX_PACKET_LENGTH to 2 * 256 * 256 + 32 = 131104.
> +*
> +* The commit making this change for gdbserver can be found here:
> +* https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=
> +* b816042e88583f280ad186ff124ab84d31fb592b
> +*/
> +#define MAX_PACKET_LENGTH 131104


Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM