Re: [Qemu-devel] global_mutex and multithread.

[Mark Burton]

> On 15 Jan 2015, at 22:41, Paolo Bonzini <address@hidden> wrote:
> 
> 
> 
> On 15/01/2015 21:53, Mark Burton wrote:
>>> Jan said he had it working at least on ARM (MusicPal).
>> 
>> yeah - our problem is when we enable multi-threads - which I dont believe 
>> Jan did…
> 
> Multithreaded TCG, or single-threaded TCG with SMP?

He mentions SMP, - I assume thats single-threaded ….

> 
>>>> One thing I wonder - why do we need to go to the extent of mutexing
>>>> in the TCG like this? Why can’t you simply put a mutex get/release on
>>>> the slow path? If the core is going to do ‘fast path’ access to the
>>>> memory, - even if that memory was IO mapped - would it matter if it
>>>> didn’t have the mutex?
>>> 
>>> Because there is no guarantee that the memory map isn't changed by a
>>> core under the feet of another.  The TLB (in particular the "iotlb") is
>>> only valid with reference to a particular memory map.
>> 
>>> 
>>> Changes to the memory map certainly happen in the slow path, but lookups
>>> are part of the fast path.  Even an rwlocks is too slow for a fast path,
>>> hence the plan of going with RCU.
>> 
>> Could we arrange the world such that lookups ‘succeed’ (the wheels
>> dont fall off) -ether getting the old value, or the new, but not getting
>> rubbish - and we still only take the mutex if we are going to make
>> alterations to the MM itself? (I have’t looked at the code around that…
>> so sorry if the question is ridiculous).
> 
> That's the definition of RCU. :)  Look at the docs in
> http://permalink.gmane.org/gmane.comp.emulators.qemu/313929 for more
> information. :)

Ahh - I see !

> 
> It's still not trivial to make it 100% correct, but at the same time
> it's not too hard to prepare something decent to play with.  Also, most
> of the work can be done with KVM so it's more or less independent from
> what you guys have been doing so far.

Yes - the issue is if we end up relying on it.
But - I see what you mean - these 2 things can ‘dovetail’ together 
“independently” - so - Jan’s patch will be good for now, and then later we can 
use RCU to make it work more generally (and more efficiently).

So - our only small problem is getting Jan’s patch to work for multi-thread :-))

Cheers

Mark.

> 
> Paolo


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