Re: [Qemu-devel] qemu-2.8-rc4 is broken

[Pavel Dovgalyuk]

> From: Alex Bennée [mailto:address@hidden
> Pavel Dovgalyuk <address@hidden> writes:
> 
> >> From: Alex Bennée [mailto:address@hidden
> >
> > Sorry, this is another problem which occurs only in icount replay mode:
> > 1. cpu_handle_exception tries to force exception when is cannot occur due to
> >    running out all the planned instructions:
> >     } else if (replay_has_exception()
> >                && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
> >         /* try to cause an exception pending in the log */
> >         cpu_exec_nocache(cpu, 1, tb_find(cpu, NULL, 0), true);
> >         *ret = -1;
> >         return true;
> >
> > 2. tb_find calls tb_gen_code, which cannot allocate new translation block
> >    and calls tb_flush (which only queues the flushing) and cpu_loop_exit
> > 3. cpu_loop_exit returns to infinite loop of cpu_exec and the condition
> >             if (cpu_handle_exception(cpu, &ret)) {
> >                 break;
> >             }
> >    is checked again causing an infinite loop.
> >
> > TB cache is not flushed because we never execute that break and real work 
> > of tb_flush
> > is made outside this loop.
> 
> I think what we need is a:
> 
> 
>   if (cpu->exit_request)
>     break;

Where this exit_request is supposed to be set?

> 
> before the cpu_handle_exception() call to ensure any queued work gets
> processed first. Can you give me you current command line so I can
> reproduce this and check the fix works?

I solved the problem using following patch:

--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -451,6 +451,10 @@ static inline bool cpu_handle_exception(CPUState *cpu, int 
*ret)
 #ifndef CONFIG_USER_ONLY
     } else if (replay_has_exception()
                && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
+        /* Break the execution loop in case of running out of TB cache.
+           This is needed to make flushing of the TB cache, because
+           real flush is queued to be executed outside the cpu loop. */
+        cpu->exception_index = EXCP_INTERRUPT;
         /* try to cause an exception pending in the log */
         cpu_exec_nocache(cpu, 1, tb_find(cpu, NULL, 0), true);
         *ret = -1;

Pavel Dovgalyuk