Re: [Qemu-devel] CPU TLB flush with multithread TCG.

[Frederic Konrad <fred.konrad@greensocs.com>]

On 11/02/2015 04:33, Alex Bennée wrote:
> Frederic Konrad <fred.konrad@greensocs.com> writes:
>
>> Hi everybody,
>>
>> In multithread tlb_flush is broken as CPUA can flush an other CPUB and
>> CPUB can be
>> executing code, and fixing this can be quite hard:
>>     * We need to exit the CPU which is flushed.
>>     * Makes sure the CPU is stopped.
>>     * Then we can flush tlb.
>> The big issues are:
>>     * Two threads can be doing a flush at the same time.
>>     * Something can restart the CPU during the flush.
>>
>> A better idea I think is that instead of flushing tlb we can put a flag
>> in CPUState such
>> as flush_request and ask the cpu to exit.
>> Then later once the CPU is exited we can flush tlbs if flush_request is set.
>> It will ensure that the CPU won't execute code as it's associated thread
>> will be
>> flushing.
>>
>> Can this work?
> Does this imply deferring the work? Surely if we don't flush when
> instructed things could break down very quickly?
>
Yes this imply deferring the work. It might be an issue as we can't exit 
the CPU
directly, they will finish their execution before flushing..

But maybe it's only a problem if the CPU flushes himself?
eg: if CPUB is executing instructions and CPUA wants to flush it: does 
that makes a
difference if the flush happen before or after the block of instruction 
executed by
CPUB?

Thanks,
Fred
>
>> Thanks,
>> Fred