Re: [Qemu-devel] Linux kernel polling for QEMU

[Fam Zheng]

On Tue, 11/29 14:27, Paolo Bonzini wrote:
> 
> 
> On 29/11/2016 14:24, Fam Zheng wrote:
> > On Tue, 11/29 12:17, Paolo Bonzini wrote:
> >>
> >>
> >> On 29/11/2016 11:32, Fam Zheng wrote:
> >>>
> >>> The kernel change will be a new prctl operation (should it be a different
> >>> syscall to extend?) to register a new type of eventfd called "idle 
> >>> eventfd":
> >>>
> >>>     prctl(PR_ADD_IDLE_EVENTFD, int eventfd);
> >>>     prctl(PR_DEL_IDLE_EVENTFD, int eventfd);
> >>>
> >>> It will be notified by kernel each time when the thread's local core has 
> >>> no
> >>> runnable threads (i.e., entering idle state).
> >>>
> >>> QEMU can then add this eventfd to its event loop when it has events to 
> >>> poll, and
> >>> watch virtqueue/linux-aio memory from userspace in the fd handlers.  
> >>> Effectiely,
> >>> if a ppoll() would have blocked because there are no new events, it could 
> >>> now
> >>> return immediately because of idle_eventfd events, and do the idle 
> >>> polling.
> >>
> >> This has two issues:
> >>
> >> * it only reports the leading edge of single_task_running().  Is it also
> >> useful to stop polling on the trailing edge?
> > 
> > QEMU can clear the eventfd right after event firing so I don't think it is
> > necessary.
> 
> Yes, but how would QEMU know that the eventfd has fired?  It would be
> very expensive to read the eventfd on each iteration of polling.

The idea is to ppoll() the eventfd together with other fds (ioeventfd and
linux-aio etc.), and in the handler, call event_notifier_test_and_clear()
followed by a polling loop for some period.

Fam

> 
> Paolo
> 
> >> * it still needs a system call before polling is entered.  Ideally, QEMU
> >> could run without any system call while in polling mode.
> >>
> >> Another possibility is to add a system call for single_task_running().
> >> It should be simple enough that you can implement it in the vDSO and
> >> avoid a context switch.  There are convenient hooking points in
> >> add_nr_running and sub_nr_running.
> > 
> > That sounds good!
> > 
> > Fam
> >