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

[Fredrik Noring <noring@nocrew.org>]

Hi Maciej,

>  Hmm, whether it works or not seems to depend on GDB version.  It looks to 
> me like we have a regression here.  Working GDB has:
> 
> (gdb) info files
> Local core dump file:
>         `/proc/kcore', file type elf32-tradlittlemips.
>         0xffffffffc0000000 - 0xfffffffffff94000 is load1
>         0xffffffff80000000 - 0xffffffff90000000 is load2
> (gdb)
> 
> Broken GDB has:
> 
> (gdb) info files
> Local core dump file:
>         `/proc/kcore', file type elf32-tradlittlemips-freebsd.
>         0xffffffffc0000000 - 0xfffffffffff94000 is load1
>         0xffffffff80000000 - 0xffffffff90000000 is load2
> (gdb)
> 
> 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! However, I don't observe the
"freebsd" BFD target with a cross-GDB version 8.1 (via v9fs in this case):

	# mipsel-linux-gdb --version | head -n1
	GNU gdb (GDB) 8.1
	# mipsel-linux-gdb -q -c /mnt/kcore
	[New process 1]
	Core was generated by `ramdisk_size=16384 crtmode=pal1 video=ps2fb:pal,640x480-32 rd_start=0x8062c000'.
	#0  0x00000000 in ?? ()
	(gdb) show gnutarget
	The current BFD target is "auto".
	(gdb) info files
	Local core dump file:
		`/mnt/kcore', file type elf32-tradlittlemips.
		0xffffffffc0000000 - 0xfffffffffffcd000 is load1
		0xffffffff80000000 - 0xffffffff80001000 is load2
		0xffffffff80010000 - 0xffffffff82000000 is load3
	(gdb) x /32i 0xffffffff80000000
	   0x80000000:	Cannot access memory at address 0x80000000
	(gdb) x /32i 0x80000000
	   0x80000000:	Cannot access memory at address 0x80000000
	(gdb)

>  Good.  You probably want to add `--adjust-vma=0x80000000' to `objdump', 
> so that addresses are right.  You can use `-b binary' with `objdump' too, 
> to avoid the extra `objcopy' step.

Perfect, thanks!

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?

Fredrik