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Re: [Qemu-devel] qemu-arm-static, multiprocess, atomic
From: |
Sergey Fedorov |
Subject: |
Re: [Qemu-devel] qemu-arm-static, multiprocess, atomic |
Date: |
Thu, 26 May 2016 15:53:38 +0300 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0 |
On 26/05/16 14:21, Paolo Bonzini wrote:
>
> On 26/05/2016 12:42, Sergey Fedorov wrote:
>> I'm afraid even our recent efforts in
>> multi-threaded TCG won't change the situation. The problem is that it
>> would require to translate somehow ARM's exclusive access monitor to x86
>> model.
>>
> The cmpxchg-based variant would work. It doesn't implement LL/SC
> exactly, but it works fine if the program is written against GCC atomic
> builtins.
I'm afraid, GCC can still generate ldrex/strex pair for
__atomic_compare_exchange():
$ cat a.c
#define atomic_cmpxchg(ptr, old,
new) \
({ \
typeof(*ptr) _old = (old), _new =
(new); \
__atomic_compare_exchange(ptr, &_old, &_new,
0, \
__ATOMIC_SEQ_CST,
__ATOMIC_SEQ_CST); \
_old; \
})
int cmpexchg(int *ptr, int old, int new)
{
return atomic_cmpxchg(ptr, old, new);
}
$ arm-linux-gnueabi-gcc -march=armv7-a -O2 -c a.c
$ arm-linux-gnueabi-objdump -d a.o
a.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <cmpexchg>:
0: e24dd008 sub sp, sp, #8
4: e58d1004 str r1, [sp, #4]
8: f57ff05f dmb sy
c: e1903f9f ldrex r3, [r0]
10: e1530001 cmp r3, r1
14: 1a000002 bne 24 <cmpexchg+0x24>
18: e180cf92 strex ip, r2, [r0]
1c: e35c0000 cmp ip, #0
20: 1afffff9 bne c <cmpexchg+0xc>
24: f57ff05f dmb sy
28: e1a00003 mov r0, r3
2c: e28dd008 add sp, sp, #8
30: e12fff1e bx lr
Kind regards,
Sergey