Re: [Qemu-devel] [PATCH v6 1/8] atomic: introduce atomic operations

[liu ping fan]

On Fri, Nov 16, 2012 at 8:03 AM, Richard Henderson <address@hidden> 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~