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

[liu ping fan]

On Sun, Nov 18, 2012 at 6:04 PM, Avi Kivity <address@hidden> wrote:
> On 11/15/2012 09:47 AM, liu ping fan wrote:
>> On Wed, Nov 14, 2012 at 5:47 PM, Paolo Bonzini <address@hidden> wrote:
>>> Il 14/11/2012 10:38, liu ping fan ha scritto:
>>>> On Tue, Nov 13, 2012 at 6:11 PM, Paolo Bonzini <address@hidden> wrote:
>>>>>>> Il 05/11/2012 06:38, Liu Ping Fan ha scritto:
>>>>>>>> From: Liu Ping Fan <address@hidden>
>>>>>>>>
>>>>>>>> If out of global lock, we will be challenged by SMP in low level,
>>>>>>>> so need atomic ops.
>>>>>>>>
>>>>>>>> This file is a wrapper of GCC atomic builtin.
>>>>>>>
>>>>>>> I still object to this.
>>>>>>>
>>>>>>> I know it enforces type-safety, but it is incomplete.  It doesn't
>>>>>>
>>>>>> Although it is incomplete, but the rest cases are rarely used.  Linux
>>>>>> faces such issue, and the "int" version is enough, so I think we can
>>>>>> borrow experience from there.
>>>>>
>>>>> One of the two places that use __sync_* require 64-bit accesses.  My
>>>> Yes, these two places are not easy to fix.
>>>
>>> Which shows that Linux's atomic_t is not suited for QEMU, in my opinion.
>>>
>>>>> RCU prototype required pointer-sized access, which you cannot make type-
>>>> But I think that your RCU prototype should rely on atomic of CPU, not
>>>> gcc‘s atomic.
>>>
>>> What's the difference?  gcc's atomic produces the same instructions as
>>> hand-written assembly (or should).
>>>
>> 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.
>>
>> And I think the broken issue is caused by vhost thread updates log,
>> while qemu read out it not atomicly, Right?
>
> For the log, 32-bit sync_fetch_and_and() is sufficient.  We only need to
> ensure no bits are lost, we don't need 64-bit atomicity.
>
Read vhost related code, just find it is quite different from kvm log.
And understand it.

Thanks and regards,
Pingfan
>
>
> --
> error compiling committee.c: too many arguments to function