Re: [Qemu-devel] [PATCH v2] TCG: Convert global variables to be TLS.

[陳韋任 <chenwj@iis.sinica.edu.tw>]

> If you're serious about multithreading TCG then I think the first
> steps are:
>  * fix existing race conditions
>  * think very hard
>  * come up with an overall design for what you're proposing

  As COREMU [1] point out, current QEMU atomic instruction emulation approach is
problematic. For example, guest application might use x86 xchg instruction to
implement spin lock/unlock (addr is a shared memory space).


      spin_unlock:                   spin_lock:
                                     
                                     try:
                                       r10 = 1;
                                       xchg addr, r10;
                                       if (r10 == 0)
                                         goto success;
      *addr = 0;                     fail:
                                       pause;
                                       if (*addr != 0)
                                         goto fail;

                                       goto try;

                                     success:

                                     
After QEMU translation, guest xchg instruction becomes

      spin_unlock:                   spin_lock:

                                     helper_lock;

      *addr = 0;                     T0 = r10;
                                     T1 = *addr;
                                     *addr = T0;
                                     r10 = T1;

                                     helper_unlock;

  You can the see the atomicity on which spin lock/unlock rely is broken.
"*addr = 0" can happened in the between of helper_lock/helper_unlock.
COREMU solve this by using a lightway software transaction memory to emulate
atomic instructions. I think this issue is quite important if we want to make
TCG multithreaded, right? Is there a better way to solve this?

Regards,
chenwj

[1]
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.187.6011&rep=rep1&type=pdf

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj