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


From: Laurent Vivier
Subject: Re: m68k: gdbstub crashing setting float register on cfv4e cpu
Date: Thu, 16 Apr 2020 13:18:51 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

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).

Thanks,
Laurent



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