Re: [PATCH] arm64: kgdb: Match pstate size with gdbserver protocol

[Daniel Thompson]

On 10/05/16 11:28, Will Deacon wrote:
> Hi Daniel,
>
> So does kgdb do something useful after this patch?

On this occasion I was debugging it on behalf of another developer 
rather than trying to use it myself to debug something so I didn't do 
much additional testing beyond sanity tested the contents of the 
register set.

Naturally it will be *totally* awesome if I get the pseudo-NMI stuff 
working perfectly. ;-)


> On Mon, May 09, 2016 at 06:39:26PM +0100, Daniel Thompson wrote:
> > Current versions of gdb do not interoperate cleanly with kgdb on arm64
> > systems because gdb and kgdb do not use the same register description.
> > This patch modifies kgdb to work with recent releases of gdb (>= 7.8.1).
> >
> > Compatibility with gdb (after the patch is applied) is as follows:
> >
> >   gdb-7.6 and earlier  Ok
> >   gdb-7.7 series       Works if user provides custom target description
> >   gdb-7.8(.0)          Works if user provides custom target description
> >   gdb-7.8.1 and later  Ok
> >
> > When commit 44679a4f142b ("arm64: KGDB: Add step debugging support") was
> > introduced it was paired with a gdb patch that made an incompatible
> > change to the gdbserver protocol. This patch was eventually merged into
> > the gdb sources:
> > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=a4d9ba85ec5597a6a556afe26b712e878374b9dd
> >
> > The change to the protocol was mostly made to simplify big-endian support
> > inside the kernel gdb stub. Unfortunately the gdb project released
> > gdb-7.7.x and gdb-7.8.0 before the protocol incompatibility was identified
> > and reversed:
> > https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=bdc144174bcb11e808b4e73089b850cf9620a7ee
> >
> > This leaves us in a position where kgdb still uses the no-longer-used
> > protocol; gdb-7.8.1, which restored the original behaviour,If was
> > released on 2014-10-29.
> >
> > I don't believe it is possible to detect/correct the protocol
> > incompatiblity which means the kernel must take a view about which
> > version of the gdb remote protocol is "correct". This patch takes the
> > view that the original/current version of the protocol is correct
> > and that version found in gdb-7.7.x and gdb-7.8.0 is anomalous.
>
> Urgh, this is filthy! Still, without a time machine, I guess there's
> little we can do about it. Can I ask you to respin the patch but with
> the rationale as a comment in the header file, and a pointer to the
> comment from the C code too, please?
>
> The code looks incorrect after this change, so we should justify how
> we've ended up in this state and not everybody looks at the git log
> for that rationale.

Will do.


Daniel.