Re: [PATCH] gdb/riscv: Add target description support

[Jim Wilson <jimw@sifive.com>]

On Sat, Feb 23, 2019 at 12:51 PM Andrew Burgess
<andrew.burgess@embecosm.com> wrote:
> I think we should be OK.  The existing CSR feature file should
> possibly be deleted, it's not actually used right now.

I added the gdb CSR feature file to qemu in my patch.  The lack of
register numbers means I had to add a table to translate the xml CSR
register numbers into actual hardware numbers inside qemu.  But this
does work.  For the subset of CSRs that both qemu and gdb know about,
I can print register values from gdb and it works.  This is an
important feature for people using system qemu to debug bootloaders,
and maybe kernels.  This is part of the reason I wrote the qemu
gdbstub support.  For user qemu, we don't allow CSR access of course,
other than the ones readable from user space like fcsr.

(gdb) target remote :1234
Remote debugging using :1234
0x0000000000001000 in ?? ()
(gdb) set debug remote 1
(gdb) info registers misa
misa           Sending packet: $pa0#01...Ack
Packet received: 2d11140000000080
Packet p (fetch-register) is supported
0x800000000014112d RV64ACDFIMSU
(gdb)

I'm not sure what adding registers numbers to the gp and fp xml files
will do to the qemu support.  I will have to test that.  Hopefully
there is no effect unless I add the new files to qemu, in which case I
might need to update the gdbstub support then to match the new
numbers.  My understanding is that the qemu copy of the files is
supposed to be the same as the gdb copy of these files, but I'm not an
expert on this.  This is true for several targets that I checked.

OpenOCD also has support for sending a csr xml file to gdb, though of
course it does not use the gdb version of the file.  It has (or
creates) ifs own csr xml file.

Jim


Jim