Re: Userspace hypercalls?

[Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Mon, 2007-08-27 at 23:01 +0300, Avi Kivity wrote:
> Luca wrote:
> >> 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.


I agree that we should start with an exit and then later add an eventfd
mechanism if it is needed.

Regards,

Anthony Liguori


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