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[Qemu-devel] Re: [patch uq/master 1/2] virtio-pci: wake up iothread on V

From: Avi Kivity
Subject: [Qemu-devel] Re: [patch uq/master 1/2] virtio-pci: wake up iothread on VIRTIO_PCI_QUEUE_NOTIFY
Date: Mon, 22 Feb 2010 16:51:46 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100120 Fedora/3.0.1-1.fc12 Thunderbird/3.0.1

On 02/22/2010 04:29 PM, Marcelo Tosatti wrote:
On Mon, Feb 22, 2010 at 04:20:52PM +0200, Avi Kivity wrote:
On 02/22/2010 03:59 PM, Marcelo Tosatti wrote:
VIRTIO_PCI_QUEUE_NOTIFY is used to inform availability of new buffers,
so wakeup the iothread to process that information immediately.

Reported-by: Amit Shah<address@hidden>
Signed-off-by: Marcelo Tosatti<address@hidden>

Index: qemu/hw/virtio-pci.c
--- qemu.orig/hw/virtio-pci.c
+++ qemu/hw/virtio-pci.c
@@ -204,6 +204,7 @@ static void virtio_ioport_write(void *op
          virtio_queue_notify(vdev, val);
+        qemu_notify_event();
virtio_queue_notify() will call ->handle_output(), which should
either do what's needed to be done, or wake up some iothread itself.
kick is used to inform either output processing, in which case
->handle_output() does what its supposed to.

But its also used to inform availability of new buffers, which is common
to all virtio devices. So what is the point pushing this to

I don't understand what this means. ->handle_output() _is_ informing the device model of new buffers. What more is needed?

Are you concerned about spurious wakeups?

Yes. Also, qemu_notify_event() is an undirected notification (wakes up all iothreads, and all devices), whereas ->handle_output() is directed (wakes up exactly what is needed).

What's the underlying problem? A new input buffer has become available, and we need to re-poll the incoming file descriptor? If so, that's best done from ->handle_output() (either by waking the iothread or calling read() itself and perhaps receiving -EAGAIN).

error compiling committee.c: too many arguments to function

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