Re: [virtio-dev] [PATCH v8] virtio_net: support for split transport header

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 29, 2022 at 09:48:33AM +0800, Jason Wang wrote:
> On Wed, Sep 28, 2022 at 9:39 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Mon, Sep 26, 2022 at 04:06:17PM +0800, Jason Wang wrote:
> > > > Jason I think the issue with previous proposals is that they conflict
> > > > with VIRTIO_F_ANY_LAYOUT. We have repeatedly found that giving the
> > > > driver flexibility in arranging the packet in memory is benefitial.
> > >
> > >
> > > Yes, but I didn't found how it can conflict the any_layout. Device can just
> > > to not split the header when the layout doesn't fit for header splitting.
> > > (And this seems the case even if we're using buffers).
> >
> > Well spec says:
> >
> >         indicates to both the device and the driver that no
> >         assumptions were made about framing.
> >
> > if device assumes that descriptor boundaries are where
> > driver wants packet to be stored that is clearly
> > an assumption.
> 
> Yes but what I want to say is, the device can choose to not split the
> packet if the framing doesn't fit. Does it still comply with the above
> description?
> 
> Thanks

The point of ANY_LAYOUT is to give drivers maximum flexibility.
For example, if driver wants to split the header at some specific
offset this is already possible without extra functionality.

Let's keep it that way.

Now, let's formulate what are some of the problems with the current way.



A- mergeable buffers is even more flexible, since a single packet
  is built up of multiple buffers. And in theory device can
  choose arbitrary set of buffers to store a packet.
  So you could supply a small buffer for headers followed by a bigger
  one for payload, in theory even without any changes.
  Problem 1: However since this is not how devices currently operate,
  a feature bit would be helpful.

  Problem 2: Also, in the past we found it useful to be able to figure out whether
  packet fits in a single buffer without looking at the header.
  For this reason, we have this text:

	If a receive packet is spread over multiple buffers, the device
	MUST use all buffers but the last (i.e. the first \field{num_buffers} -
	1 buffers) completely up to the full length of each buffer
	supplied by the driver.

  if we want to keep this optimization and allow using a separate
  buffer for headers, then I think we could rely on the feature bit
  from Problem 1 and just make an exception for the first buffer.
  Also num_buffers is then always >= 2, maybe state this to avoid
  confusion.

  



B- without mergeable, there's no flexibility. In particular, there can
not be uninitialized space between header and data. If we had flexibility here, this could be
helpful for alignment, security, etc.
Unfortunately, our hands are tied:


	\field{len} is particularly useful
	for drivers using untrusted buffers: if a driver does not know exactly
	how much has been written by the device, the driver would have to zero
	the buffer in advance to ensure no data leakage occurs.

	For example, a network driver may hand a received buffer directly to
	an unprivileged userspace application.  If the network device has not
	overwritten the bytes which were in that buffer, this could leak the
	contents of freed memory from other processes to the application.


so all buffers have to be initialized completely up to the reported
used length.

It could be that the guarantee is not relevant in some use-cases.
We would have to specify that this is an exception to the rule,
explain that drivers must be careful about information leaks.
Let's say there's a feature bit that adds uninitialized space
somewhere. How much was added? We can find out by parsing the
packet but once you start doing that just to assemble the skb
you have already lost performance.
So lots of spec work, some security risks, and unclear performance.




Is above a fair summary?



If yes I would say let's address A, but make sure we ask drivers
to clear the new feature bit if there's no mergeable
(as opposed to e.g. failing probe) so we can add
support for !mergeable down the road.





-- 
MST