Re: [Qemu-devel] [6391] Fix nographic mode and VNC

[Anthony Liguori <anthony@codemonkey.ws>]

Paul Brook wrote:
>>>> A second solution is to use a polling select() in cpu_exec.  Since
>>>> you're adding a system call (and a rather heavy one) in the fast path,
>>>> this is going to likely hurt TCG performance.
>>>>         
>>> This won't work. If the guest really is in a tight loop then TB chaining
>>> means it will never exit translated code.
>>>       
>> But then signal delivery wouldn't either, right?  That suggests that if
>> the guest is in a tight loop right now, QEMU will freeze.
>>     
>
> No. The signal handler calls cpu_interrupt, which unlinks the TBs.
>   

Which isn't thread safe.  Okay, then you also need to send a signal to 
the TCG thread.

Regards,

Anthony Liguori

>> There's a fair bit of code that is safe to run along side of TCG.  If we
>> separate the locking for the device model code from every thing (the
>> monitor, vnc, sdl, etc.), then we could still let QEMU be responsive
>> even in such a condition.
>>     
>
> Maybe. You risk having to put an SMP safe lock in the MMIO handler, which 
> would probably do bad things to performance. Many of the embedded targets 
> don't have DMA capable peripherals, so we want to avoid making MMIO too 
> expensive. kvm is a bit different because MMIO is already horribly expensive.
>
> Paul
>