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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 17:32:05 +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 05:29 PM, Anthony Liguori wrote:
On 02/22/2010 09:16 AM, Marcelo Tosatti wrote:
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).
Yes. Sure, perhaps calling read() itself is appropriate, and i see
your point that>handle_output contains more context for a smarter

But one can argue thats an improvement on top of a dumb wakeup.

Spurious calls to qemu_notify_event() also make it difficult to tell when it's actually necessary to call qemu_notify_event() vs. when it's just something that doesn't hurt.

One improvement in this area would be to add a context parameter (which eventually resolves to the underlying thread). Currently we'd ignore it since we have just one iothread, but it would serve to document what's being polled, and later direct the wakeup to the correct thread.

error compiling committee.c: too many arguments to function

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