Re: [PATCH 0/3] gdbstub: add support for switchable endianness

[Peter Maydell]

On Fri, 27 Aug 2021 at 15:49, Changbin Du <changbin.du@gmail.com> wrote:
>
> On Tue, Aug 24, 2021 at 10:11:14AM +0100, Peter Maydell wrote:
> > On Tue, 24 Aug 2021 at 00:05, Changbin Du <changbin.du@gmail.com> wrote:
> > >
> > > On Mon, Aug 23, 2021 at 04:30:05PM +0100, Peter Maydell wrote:
> > > > changes to be more capable of handling dynamic target changes
> > > > (this would also help with eg debugging across 32<->64 bit switches);
> > > > as I understand it that gdb work would be pretty significant,
> > > > and at least for aarch64 pretty much nobody cares about
> > > > big-endian, so nobody's got round to doing it yet.
> > > >
> > > Mostly we do not care dynamic target changes because nearly all OS will 
> > > setup
> > > endianness mode by its first instruction. And dynamic changes for gdb is 
> > > hard
> > > since the byte order of debugging info in elf is fixed. And currently the 
> > > GDB
> > > remote protocol does not support querying endianness info from remote.
> > >
> > > So usually we needn't change byte order during a debug session, but we 
> > > just want
> > > the qemu gdbstub can send data in and handle data it received in right 
> > > byte order.
> > > This patch does this work with the help of users via the option 
> > > 'endianness='.
> >
> > I'm not a huge fan of putting in workarounds that deal with the
> > problem for specific cases and require users to tweak options settings,
> > rather than solving the problem in a more general way that would
> > let it all Just Work for all cases.
> >
> Probably we can add a new callback 'gdb_get_endianness' for CPUClass, and use
> this callback to determine if bswap is needed every time we read/write cpu
> registers. What's your thought?

I think that you need to start by talking to the gdb folks about
how debugging a dynamic endianness target should work. Fixing
this probably goes something like:
 * agree on design for how dynamic endianness, 32-64 mode changes,
   etc, should be handled by gdb
 * make gdb changes
 * document required gdbstub protocol enhancements (ie how the stub
   tells gdb about endianness changes, whether this changes how we
   send register information, memory data, etc)
 * implement those changes in QEMU

You seem to be trying to start with the final step, not the first one :-)

-- PMM