On Wed, 11/30 09:38, Stefan Hajnoczi wrote: > On Wed, Nov 30, 2016 at 01:42:14PM +0800, Fam Zheng wrote: > > On Tue, 11/29 20:43, Stefan Hajnoczi wrote: > > > On Tue, Nov 29, 2016 at 1:24 PM, Fam Zheng <f...@redhat.com> wrote: > > > > On Tue, 11/29 12:17, Paolo Bonzini wrote: > > > >> On 29/11/2016 11:32, Fam Zheng wrote: > > > >> * 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! > > > > > > With this solution QEMU can either poll virtqueues or the host kernel > > > can poll NIC and storage controller descriptor rings, but not both at > > > the same time in one thread. This is one of the reasons why I think > > > exploring polling in the kernel makes more sense. > > > > That's true. I have one question though: controller rings are in a different > > layer in the kernel, I wonder what the syscall interface looks like to ask > > kernel to poll both hardware rings and memory locations in the same loop? > > It's > > not obvious to me after reading your eventfd patch. > > Current descriptor ring polling in select(2)/poll(2) is supported for > network sockets. Take a look at the POLL_BUSY_LOOP flag in > fs/select.c:do_poll(). If the .poll() callback sets the flag then it > indicates that the fd supports busy loop polling. > > The way this is implemented for network sockets is that the socket looks > up the napi index and is able to use the NIC driver to poll the rx ring. > Then it checks whether the socket's receive queue contains data after > the rx ring was processed. > > The virtio_net.ko driver supports this interface, for example. See > drivers/net/virtio_net.c:virtnet_busy_poll(). > > Busy loop polling isn't supported for block I/O yet. There is currently > a completely independent code path for O_DIRECT synchronous I/O where > NVMe can poll for request completion. But it doesn't work together with > asynchronous I/O (e.g. Linux AIO using eventfd with select(2)/poll(2)).
This makes perfect sense now, thanks for the pointers! > > > > The disadvantage of the kernel approach is that you must make the > > > ppoll(2)/epoll_wait(2) syscall even for polling, and you probably need > > > to do eventfd reads afterwards so the minimum event loop iteration > > > latency is higher than doing polling in userspace. > > > > And userspace drivers powered by dpdk or vfio will still want to do polling > > in > > userspace anyway, we may want to take that into account as well. > > vfio supports interrupts so it can definitely be integrated with > adaptive kernel polling (i.e. poll for a little while and then wait for > an interrupt if there was no event). > > Does dpdk ever use interrupts? Yes, interrupt mode is supported there. For example see the intx/msix init code in drivers/net/ixgbe/ixgbe_ethdev.c:ixgbe_dev_start(). Fam