[Qemu-devel] Unable to set register on qemu-system-sparc64 via gdbstub

[Mark Cave-Ayland]

Hi all,

It looks as if the recent gdbstub code rework has broken the ability to set 
registers
under qemu-system-sparc64:

$ sparc64-linux-gdb obj-sparc64/openbios-builtin.elf.nostrip
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Reading symbols from obj-sparc64/openbios-builtin.elf.nostrip...done.
(gdb) target remote :1234
Remote debugging using :1234
0x000001fff0000020 in ?? ()
(gdb) info regi $g1
g1             0x0      0
(gdb) set $g1 = 0x55
Could not write register "g1"; remote failure reply 'E00'
(gdb)

I managed to narrow this down to the recent gdbstub rework, and in particular 
to this
patch:

commit 62b3320bddd79c050553ea7f81f20c6d3b401ce3
Author: Jon Doron <address@hidden>
Date:   Wed May 29 09:41:36 2019 +0300

    gdbstub: Implement set register (P pkt) with new infra

    Signed-off-by: Jon Doron <address@hidden>
    Message-Id: <address@hidden>
    Signed-off-by: Alex Bennée <address@hidden>

Tracing through I see that the problem occurs because of this code in gdbstub's
handle_set_reg:

static void handle_set_reg(GdbCmdContext *gdb_ctx, void *user_ctx)
{
    int reg_size;

    if (!gdb_has_xml) {
        put_packet(gdb_ctx->s, "E00");
        return;
    }

    ...
    ...
}

Because SPARC doesn't have any GDB XML files then this check always fails which 
is
why the E00 error code is being returned.

In fact if I simply comment out the above check then everything appears to work
again, however I'm not sure that this is the correct fix because there are 
several
other references to gdb_has_xml remaining in the file?


ATB,

Mark.