Re: [Qemu-devel] qemu-arm-static, multiprocess, atomic

[Sergey Fedorov]

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