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Re: [Qemu-devel] qemu-2.8-rc4 is broken
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] qemu-2.8-rc4 is broken |
Date: |
Mon, 23 Jan 2017 09:38:35 +0000 |
User-agent: |
mu4e 0.9.19; emacs 25.1.91.4 |
Pavel Dovgalyuk <address@hidden> writes:
>> 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?
Ahh my mistake. Currently it is a global exit_request (becoming a
per-cpu exit_request when MTTCG is merged). It's set by qemu_cpu_kick()
when work is queued up, in this case the tb_flush async work.
>> 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;
I wonder if it worth renaming EXCP_INTERRUPT? I always get it confused
with a guest interrupt. But the effect is the same as we set it on an
exit_request.
--
Alex Bennée