Re: Userspace hypercalls?

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Luca wrote:
> On 8/27/07, Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:
>   
>> Anthony Liguori wrote:
>>     
>>> On Mon, 2007-08-27 at 20:36 +0300, Avi Kivity wrote:
>>>       
>>>>  userspace can
>>>> attach a signal to the eventfd if it wants a synchronous exit (does
>>>> eventfd allow fcntl(F_SETOWN)?)
>>>>         
>>> Which would address the latency issue nicely.  Looking at the fs code,
>>> it looks like eventfd shouldn't have to do anything special for it.
>>>       
>> I'm not sure now.  Which thread will be selected for accepting the
>> signal?
>>     
>
> It's not specified.
>
>   

So that option's down.

>> if it isn't guaranteed to be the current thread, we're back
>> with scheduler involvement, and possibly cacheline bouncing.
>>     
>
> It's possible to use pthread_sigmask() to block the signal on all
> threads but one. But this would require changing the rest of the
> emulator; why not just select() the fd in a dedicated thread?
>   

When the guest issues that hypercall, it really wants the I/O to start.  
If it's a separate thread, the scheduler could choose to let the guest 
execute and keep the I/O thread waiting (CFS isn't likely to do that, 
but it's possible).

The scheduler could also choose to run the I/O thread on a different 
processor, and now all the data structures carefully loaded into cache 
by the guest need to be bounced to the processor running the I/O thread.

I think the best policy here is to cover our asses and allow userspace 
to choose which method it wants to use; and we should start with the 
guest exiting to userspace to avoid the costs I mentioned.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.


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