[Xen-devel] Re: [kvm-devel] [PATCH RFC 1/3] virtio infrastructure

[Rusty Russell <rusty@xxxxxxxxxxxxxxx>]

On Sun, 2007-06-03 at 08:28 +0300, Avi Kivity wrote:
> Sure, 'used' is important (how else do you get the packet size on
> receive?)

Well, the sender can prepend the length, or for networking just leave
Linux to trim packets.  It's the trust issue which bugs me.

> ,  I'm just bitching about the linear scan.

If you're doing interrupt mitigation on a high-bandwidth device, I think
it's optimal.  And if you're not, you obviously don't care 8)

That said, thousands of descriptors may not be insane...

> Well, if you have 256 slots per direction, you will scan 4KB of memory
> per interrupt (256 slots x 2 directions x 8 bytes).  You may need a
> queue length greater than 256 for a high bandwidth interface, if you
> want to reduce your interrupt rate, or if your guest is scheduled out
> for lengthy periods (say, a millisecond).

Hmm... Perhaps I should move the used arrays into the "struct
virtio_device" and guarantee that the id (returned by add_*buf) is an
index into that.  Then we can trivially add a corresponding bit array.

This also makes the "id" easier to use from the driver side: if we add a
"max_id" field it can size its own arrays if it wants to.  Gets rid of
the linked-list in the block driver...

Thoughts?
Rusty.


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