Re: [net PATCH v5 6/6] virtio_net: XDP support for adjust_head

On 17-01-21 06:51 PM, Jason Wang wrote:

On 2017年01月21日 01:48, Michael S. Tsirkin wrote:

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On Fri, Jan 20, 2017 at 04:59:11PM +0000, David Laight wrote:

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From: Michael S. Tsirkin

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Sent: 19 January 2017 21:12

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On 2017?01?18? 23:15, Michael S. Tsirkin wrote:

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On Tue, Jan 17, 2017 at 02:22:59PM -0800, John Fastabend wrote:

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Add support for XDP adjust head by allocating a 256B header region
that XDP programs can grow into. This is only enabled when a XDP
program is loaded.

In order to ensure that we do not have to unwind queue headroom push
queue setup below bpf_prog_add. It reads better to do a prog ref
unwind vs another queue setup call.

At the moment this code must do a full reset to ensure old buffers
without headroom on program add or with headroom on program removal
are not used incorrectly in the datapath. Ideally we would only
have to disable/enable the RX queues being updated but there is no
API to do this at the moment in virtio so use the big hammer. In
practice it is likely not that big of a problem as this will only
happen when XDP is enabled/disabled changing programs does not
require the reset. There is some risk that the driver may either
have an allocation failure or for some reason fail to correctly
negotiate with the underlying backend in this case the driver will
be left uninitialized. I have not seen this ever happen on my test
systems and for what its worth this same failure case can occur
from probe and other contexts in virtio framework.

Signed-off-by: John Fastabend<redacted>

I've been thinking about it - can't we drop
old buffers without the head room which were posted before
xdp attached?

Avoiding the reset would be much nicer.

Thoughts?

As been discussed before, device may use them in the same time so it's not
safe. Or do you mean detect them after xdp were set and drop the buffer
without head room, this looks sub-optimal.

Thanks

Yes, this is what I mean.  Why is this suboptimal? It's a single branch
in code. Yes we might lose some packets but the big hammer of device
reset will likely lose more.

Why not leave let the hardware receive into the 'small' buffer (without
headroom) and do a copy when a frame is received.
Replace the buffers with 'big' ones for the next receive.
A data copy on a ring full of buffers won't really be noticed.

    David

I like that. John?

This works, I prefer this only if it uses simpler code (but I suspect) than reset.

Thanks

Before the reset path I looked at doing this but it seems to require tracking
if a buffer had headroom on a per buffer basis. I don't see a good spot to
put a bit like this? It could go in the inbuf 'ctx' added by virtqueue_add_inbuf
but I would need to change the current usage of ctx which in the mergeable case
at least is just a simple pointer today. I don't like this because it
complicates the normal path and the XDP hotpath.

Otherwise we could somehow mark the ring at the point where XDP is enabled so
that it can learn when a full iteration around the ring. But I can't see a
simple way to make this work either.

I don't know the reset look straight forward to me and although not ideal is
fairly common on hardware based drivers during configuration changes. I'm open
to any ideas on where to put the metadata to track headroom though.

Thanks,
John