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

From: Anthony Liguori
Subject: [Qemu-devel] Re: [patch uq/master 1/2] virtio-pci: wake up iothread on VIRTIO_PCI_QUEUE_NOTIFY
Date: Mon, 22 Feb 2010 09:29:15 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0

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.


Anthony Liguori

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