Re: [Qemu-devel] [RFC PATCH] target-arm: protect cpu_exclusive_*.

[Alexander Graf <agraf@suse.de>]

On 17.12.14 11:31, Mark Burton wrote:
> 
>> On 17 Dec 2014, at 11:28, Alexander Graf <agraf@suse.de> wrote:
>>
>>
>>
>> On 17.12.14 11:27, Frederic Konrad wrote:
>>> On 16/12/2014 17:37, Peter Maydell wrote:
>>>> On 16 December 2014 at 09:13,  <fred.konrad@greensocs.com> wrote:
>>>>> From: KONRAD Frederic <fred.konrad@greensocs.com>
>>>>>
>>>>> This adds a lock to avoid multiple exclusive access at the same time
>>>>> in case of
>>>>> TCG multithread.
>>> Hi Peter,
>>>
>>>> This feels to me like it's not really possible to review on
>>>> its own, since you can't see how it fits into the design of
>>>> the rest of the multithreading support.
>>> true the only thing we observe is that it didn't change anything right now.
>>>
>>>> The other approach here rather than having a pile of mutexes
>>>> in the target-* code would be to have TCG IR support for
>>>> "begin critical section"/"end critical section". Then you
>>>> could have the main loop ensure that no other CPU is running
>>>> at the same time as the critical-section code. (linux-user
>>>> already has an ad-hoc implementation of this for the
>>>> exclusives.)
>>>>
>>>> -- PMM
>>>>
>>> What do you mean by TCG IR?
>>
>> TCP ops. The nice thing is that TCG could translate those into
>> transactions if the host supports them as well.
>>
> 
> Hows that different in reality from what we have now?
> Cheers
> Mark.

The current code can't optimize things in TCG. There's a good chance
your TCG host implementation can have an optimization pass that creates
host cmpxchg instructions or maybe even transaction blocks out of the
critical sections.


Alex