Re: [RFC PATCH 00/17] virtual-bus

From: Avi Kivity
Date: Fri Apr 03 2009 - 07:03:42 EST


Gerd Hoffmann wrote:
Avi Kivity wrote:
There is no choice. Exiting from the guest to the kernel to userspace
is prohibitively expensive, you can't do that on every packet.

I didn't look at virtio-net very closely yet. I wonder why the
notification is that a big issue though. It is easy to keep the number
of notifications low without increasing latency:

Check shared ring status when stuffing a request. If there are requests
not (yet) consumed by the other end there is no need to send a
notification. That scheme can even span multiple rings (nics with rx
and tx for example).

If the host is able to consume a request immediately, and the guest is not able to batch requests, this breaks down. And that is the current situation.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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