On 2012-11-14 23:47, liu ping fan wrote: > Probably I made a mistake here, in vhost, log = > __sync_fetch_and_and(from, 0) is used to fetch 64bits atomically in > the case 32bits qemu running on 64bits linux. Right? But how can > we read 32bits twice in atomic? Seem that no instruction like "_lock > xchg" for this ops. So I guess _sync_fetch_and_and() based on > something like spinlock.
... or for gcc 4.7 and later, log = __atomic_load_n(from, memory_model) For i386, we will not perform 2 32-bit reads of course. Paulo suggests using cmpxchg8b, but that's a tad slow. Instead we'll perform a 64-bit read into either the fpu or the sse units, and from there copy the data wherever it's needed. Such 64-bit aligned reads are guaranteed to be atomic for i586 (pentium) and later. For other 32-bit architectures other possibilities exist. Recent arm can use its ldrexd insn. Many of the 32-bit linux architectures have special kernel entry points or schemes to perform atomic operations. These are generally based on the assumption of a single-processor system, and are arranged to either disable interrupts or notice that no interrupt occurred, while executing a code region. As an ultimate fallback, yes we would use locks. But none of the host architectures that QEMU supports needs to do so. r~