Re: m68k: gdbstub crashing setting float register on cfv4e cpu

[Laurent Vivier]

Le 16/04/2020 à 22:03, Pierre Muller a écrit :
> Le 16/04/2020 à 13:18, Laurent Vivier a écrit :
>> Le 14/04/2020 à 18:56, Alex Bennée a écrit :
>>>
>>> Philippe Mathieu-Daudé <address@hidden> writes:
>>>
>>>> gdbstub/m68k seems broken with floats, previous to refactor commit
>>>> a010bdbe719 ("extend GByteArray to read register helpers").
>>>>
>>>> HEAD at 6fb1603aa2:
>>>>
>>>> $ qemu-system-m68k -s -S -cpu cfv4e
>>>>
>>>> ---[GUEST]---
>>>>
>>>> (gdb) set architecture m68k:cfv4e
>>>> The target architecture is assumed to be m68k:cfv4e
>>>> (gdb) target remote 172.17.0.1:1234
>>>> Remote debugging using 172.17.0.1:1234
>>>> (gdb) info float
>>>> fp0            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fp1            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fp2            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fp3            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fp4            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fp5            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fp6            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fp7            -nan(0xfffffffffff7f) (raw 0xffffffffffffff7f)
>>>> fpcontrol      0x0                 0
>>>> fpstatus       0x0                 0
>>>> fpiaddr        0x0                 0x0
>>>> (gdb) set $fp0=1
>>>> Remote communication error.  Target disconnected.: Connection reset by
>>>> peer.
>>>
>>> With my sha1 debugging test case I get different results depending on
>>> the cpu type:
>>>
>>>   /home/alex/lsrc/qemu.git/tests/guest-debug/run-test.py --gdb 
>>> /home/alex/src/tools/binutils-gdb.git/builds/all/install/bin/gdb --qemu 
>>> /home/alex/lsrc/qemu.git/builds/user.static/m68k-linux-user/qemu-m68k 
>>> --qargs "" --bin tests/tcg/m68k-linux-user/sha1 --test
> /home/alex/lsrc/qemu.git/tests/tcg/multiarch/gdbstub/sha1.py
>>>   GNU gdb (GDB) 10.0.50.20200414-git
>>>   Copyright (C) 2020 Free Software Foundation, Inc.
>>>   License GPLv3+: GNU GPL version 3 or later 
>>> <http://gnu.org/licenses/gpl.html>
>>>   This is free software: you are free to change and redistribute it.
>>>   There is NO WARRANTY, to the extent permitted by law.
>>>   Type "show copying" and "show warranty" for details.
>>>   This GDB was configured as "x86_64-pc-linux-gnu".
>>>   Type "show configuration" for configuration details.
>>>   For bug reporting instructions, please see:
>>>   <http://www.gnu.org/software/gdb/bugs/>.
>>>   Find the GDB manual and other documentation resources online at:
>>>       <http://www.gnu.org/software/gdb/documentation/>.
>>>
>>>   For help, type "help".
>>>   Type "apropos word" to search for commands related to "word"...
>>>   Executed .gdbinit
>>>   Reading symbols from tests/tcg/m68k-linux-user/sha1...
>>>   Remote debugging using localhost:1234
>>>   warning: Register "fp0" has an unsupported size (96 bits)
>>>   warning: Register "fp1" has an unsupported size (96 bits)
>>>   warning: Register "fp2" has an unsupported size (96 bits)
>>>   warning: Register "fp3" has an unsupported size (96 bits)
>>>   warning: Register "fp4" has an unsupported size (96 bits)
>>>   warning: Register "fp5" has an unsupported size (96 bits)
>>>   warning: Register "fp6" has an unsupported size (96 bits)
>>>   warning: Register "fp7" has an unsupported size (96 bits)
>>>   Remote 'g' packet reply is too long (expected 148 bytes, got 180 bytes):
> 000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000408009f000000000800003407fff0000ffffffffffffffff7fff0000ffffffffffffffff7fff0000ffffffffffffffff7fff0000ffffffffffffffff7fff0000ffffffffffffffff7fff0000ffffffffffffffff7fff0000ffffffffffffffff7fff0000ffffffffffffffff000000000000000000000000
>>
>> This is a bug in GDB that doesn't support 96bit float registers of 680x0
>> but only 64bit registers of coldfire.
>>
>> There was a rework of GDB in the past that has broken that and no one
>> noticed. I bisected and found the commit but it was really too complex
>> and difficult to fix.
>>
>> To be able to debug remotely m68k I use gdb from etch-m68k in a chroot
>> (or from real hardware).
> 
>   I do have a fix for gdb-8.3 release: it works for me.
> See patch below,
> 
>   You could test it out on other versions,
> changes to m68k-tdep.c are not that big in recent GDB releases.
>   I use it with a locally modified qemu to try to support FPU
> exceptions for m68k FPU.
>   But I never found the time nor the enery to try to submit those
> to qemu-devel, especially after viewing what happened to a similar
> attempt for powerpc hardware fpu support.
> See "[RFC PATCH v2] target/ppc: Enable hardfloat for PPC" thread, up to
> https://lists.nongnu.org/archive/html/qemu-ppc/2020-03/msg00006.html

But why didn't you submit your patch to gdb?

Thanks,
Laurent