On Fri, Nov 16, 2012 at 8:03 AM, Richard Henderson <r...@twiddle.net> wrote: > 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. > Very appreciate for you so detailed description.
Regards, Pingfan > > r~
