Re: [RFC v2] MIPS: R5900: Workaround exception NOP execution bug (FLX05)

["Maciej W. Rozycki" <macro@mips.com>]

Hi Fredrik,

> > Notice the different BFD target, `elf32-tradlittlemips-freebsd'.  You're 
> > supposed to be able to override it with `set gnutarget', but that doesn't 
> > seem to impress GDB, e.g.:
> > 
> > (gdb) show gnutarget
> > The current BFD target is "auto".
> > (gdb) set gnutarget elf32-tradlittlemips
> > (gdb) show gnutarget
> > The current BFD target is "elf32-tradlittlemips".
> > (gdb) info files
> > Local core dump file:
> >         `/home/mjr/src/kcore', file type elf32-tradlittlemips-freebsd.
> >         0xffffffffc0000000 - 0xfffffffffff94000 is load1
> >         0xffffffff80000000 - 0xffffffff90000000 is load2
> > (gdb)
> > 
> > I'll see if I can track down what is going on here.
> 
> Thank you for taking a closer look at GDB!

 You are welcome, however it's my duty as a MIPS/GDB port maintainer.

> However, I don't observe the
> "freebsd" BFD target with a cross-GDB version 8.1 (via v9fs in this case):

 The likely cause is my development GDB builds use `--enable-targets=all' 
and your GDB configuration probably does not include secondary targets.  
Still I find it wrong that an incorrect BFD target is chosen, and then 
that an override does not work either (and especially that accessing 
memory in a core file seems completely broken in recent GDB versions).

> By the way, what about presenting misaligned SQ instructions like
> 
> 	# mipsel-linux-gdb -q busybox
> 	Reading symbols from busybox...(no debugging symbols found)...done.
> 	(gdb) set architecture mips:5900
> 	The target architecture is assumed to be mips:5900
> 	(gdb) x /i 0x4036b0
> 	   0x4036b0:	sq	v1,-6085(zero)
> 
> as RDHWR, which is the interpretation with Linux?

 Hmm, I have mixed feelings about it as RDHWR is not an R5900 instruction.  
Likewise we don't disassemble it, and neither we do LL, SC, SYNC, etc. say 
with `mips:3000' even though Linux will emulate them.  I feel this kind of 
stuff does not belong to instruction aliases either (compare `objdump' 
disassembly of any serious program with and without `-M no-aliases').

 I do recognize value for users here though, so perhaps an extra `-M' 
option would be due here, such as `-M linux-emulation' or suchlike, to 
cover all emulated instructions.  I'll think about it.  Please file an 
enhancement request in sourceware.org Bugzilla if it's something you are 
keen having (feel free to cc me; I'm <macro@linux-mips.org> there).

  Maciej