Kernel crosscompilers

[Christophe Leroy]

Le 08/05/2021 à 08:46, Christophe Leroy a écrit :
> 
> 
> Le 06/05/2021 à 16:17, Arnd Bergmann a écrit :
>> On Thu, May 6, 2021 at 2:42 PM Christophe Leroy
>> <christophe.leroy at csgroup.eu> wrote:
>>>
>>> Hello Arnd,
>>>
>>> May I ask you whether you plan to build cross compilers based on GCC 11.1 at
>>> https://mirrors.edge.kernel.org/pub/tools/crosstool/ ?
>>
>> Hi Christophe,
>>
>> I've built a snapshot a few days before the release, that one is
>> identical to 11.1
>> except for the reported version number. I've tried to ask around for
>> help testing
>> this, but so far I have not heard from anyone.
>>
>> Building a new set of compilers takes around a day on my build box, so I want
>> to make sure I don't have to do it more often than necessary. If you are able
>> to give the binaries a spin, preferably on a ppc64le or arm64 host, please let
>> me know how it goes and I'll rebuilt them on the release tag.
>>
> 
> Hi Arnd,
> 
> I don't have any ppc or arm host I can build on.
> I'm building on x86 for powerpc embedded boards.
> 
> I have tried your GCC 11 snapshot, I get something booting but it crashes when launching init.
> 
> [    7.368410] init[1]: bad frame in sys_sigreturn: 7fb2fd60 nip 001083cc lr 001083c4
> [    7.376283] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> [    7.383680] CPU: 0 PID: 1 Comm: init Not tainted 5.12.0-s3k-dev-16316-g9e799d5df185 #5054
> [    7.391767] Call Trace:
> [    7.394174] [c9023db0] [c00211e8] panic+0x130/0x304 (unreliable)
> [    7.400112] [c9023e10] [c0024e68] do_exit+0x874/0x910
> [    7.405104] [c9023e50] [c0024f80] do_group_exit+0x40/0xc4
> [    7.410440] [c9023e60] [c0033334] get_signal+0x1d8/0x93c
> [    7.415689] [c9023ec0] [c0007f34] do_notify_resume+0x6c/0x314
> [    7.421369] [c9023f20] [c000d580] syscall_exit_prepare+0x120/0x184
> [    7.427479] [c9023f30] [c001101c] ret_from_syscall+0xc/0x28
> 
> Something is going wrong with asm goto output. I implemented get_user() helpers with asm goto this 
> cycle (commit 5cd29b1fd3e8). I tested it with CLANG before submitting, it was working.
> 
> Seems like there is something wrong with it with GCC11. When forcing CONFIG_CC_HAS_ASM_GOTO_OUTPUT 
> to 'n', the kernel boots ok.
> 

I found the problem, that's due to r10 register being reused by GCC in the copy loop below:

   10:	7d 09 03 a6 	mtctr   r8
   14:	80 ca 00 00 	lwz     r6,0(r10)
   18:	80 ea 00 04 	lwz     r7,4(r10)
   1c:	90 c9 00 08 	stw     r6,8(r9)
   20:	90 e9 00 0c 	stw     r7,12(r9)
   24:	39 0a 00 08 	addi    r8,r10,8
   28:	39 29 00 08 	addi    r9,r9,8
=>2c:	81 4a 00 08 	lwz     r10,8(r10)
   30:	81 6a 00 0c 	lwz     r11,12(r10)
   34:	91 49 00 08 	stw     r10,8(r9)
   38:	91 69 00 0c 	stw     r11,12(r9)
   3c:	39 48 00 08 	addi    r10,r8,8
   40:	39 29 00 08 	addi    r9,r9,8
   44:	42 00 ff d0 	bdnz    14 <__unsafe_restore_general_regs+0x14>

earlyclobber modifier is missing in the CONFIG_CC_HAS_ASM_GOTO_OUTPUT version of __get_user_asm2_goto().

This is not a GCC bug, with that fixed my board is working OK.

Christophe