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[Qemu-devel] Re: [PATCH] qemu/virtio-net: remove wrong s/g layout assump

From: Anthony Liguori
Subject: [Qemu-devel] Re: [PATCH] qemu/virtio-net: remove wrong s/g layout assumptions
Date: Tue, 24 Nov 2009 15:04:47 -0600
User-agent: Thunderbird (X11/20090825)

Michael S. Tsirkin wrote:
On Tue, Nov 24, 2009 at 01:50:25PM -0600, Anthony Liguori wrote:
Michael S. Tsirkin wrote:
virtio net currently assumes that the first s/g element it gets is
always virtio net header. This is wrong.
There should be no assumption on sg boundaries.  For example, the guest
should be able to put the virtio_net_hdr in the front of the skbuf data
if there is room.  Get rid of this assumption, properly consume space
from iovec, always.
Practically speaking, we ought to advertise a feature bit to let a kernel know that we are no longer broken.

Otherwise, there are a ton of old userspaces that will break with new guests.

My thinking is, first of all let's fix the bug.
We'll add a feature bit when or if some guest wants to use it.
Maybe this will be 100 years down the road when all old userspace
has died a natural death :)
Makes sense?

I don't think it's useful to do this without adding a feature bit. If we don't add a feature bit, the guest kernel cannot rely on this behavior so it means by definition this is dead code.


Anthony Liguori

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