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

[Fam Zheng <famz@redhat.com>]

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.

> 
> * 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