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

[Jim Wilson <jimw at sifive dot com>]

On Tue, Feb 26, 2019 at 10:22 AM Andrew Burgess
<andrew.burgess@embecosm.com> wrote:
>
> * Jim Wilson <jimw@sifive.com> [2019-02-26 09:26:04 -0800]:
>
> > On Mon, Feb 25, 2019 at 9:02 PM Joel Brobecker <brobecker@adacore.com> wrote:
> > > I think if QEMU sends an XML with the various register description,
> > > then whatever numbering GDB uses by default will no longer apply,
> > > and so things should just-work(tm) regardless of what GDB decided
> > > to do in terms of register numbering.
> >
> > Yes, it shouldn't affect qemu until we try to copy the new gdb xml
> > files into qemu, at which point we might need to update the qemu
> > gdbstub support to work with the changed register numbers.  We can
> > worry about this later.  This issues doesn't need to delay any gdb
> > work.
>
> Jim, if you're happy then I'll go ahead and merge the fix-up patch.
>
> I'll summarise the changes in the patch, and what impact I think they
> will have now I've had a look at the QEMU code (all these changes are
> identical for 32 and 64 bit)...
>
>   (1) Added forced register number for register 'zero'.  This will
>   have no impact the default register numbering before had the
>   x-registers numbered from 0.  I added this just to make the
>   numbering explicit.
>
>   (2) Added forced register number 33 to the first floating pointer
>   register ($ft0).  Again, this should have no impact as the
>   f-registers were traditionally numbered after the 32 x-registers and
>   the program-counter.
>
>   (3) Renumbered fflags, frm, and fcsr as 66, 67, and 68.  This is
>   where the issues will appear for QEMU, Jim's QEMU patch had adopted
>   the "new" default numbering which placed these registers after the
>   floating point registers (so they had become 65, 66, and 67).
>
> If we want backward compatibility then we should merge this GDB patch,
> and fix QEMU asap to avoid having two incompatible versions in the
> wild.
>
> What I don't understand about all this is why QEMU appears to be
> discarding one of the big benefits of xml register descriptions; the
> ability to disconnect their register numbering from GDB's register
> numbering.
>
> Jim: I think your comments above indicate you want my fix merged, but
> if you could just confirm then I'll get it merged.
>
> Thanks,
> Andrew