Re: [PATCH net] virtio-net: unbreak cusmed packet for small buffer XDP

[Jason Wang]

On 2017å06æ28æ 11:31, Michael S. Tsirkin wrote:
> On Wed, Jun 28, 2017 at 10:45:18AM +0800, Jason Wang wrote:
> > On 2017å06æ28æ 10:17, Michael S. Tsirkin wrote:
> > > On Wed, Jun 28, 2017 at 10:14:34AM +0800, Jason Wang wrote:
> > > > On 2017å06æ28æ 10:02, Michael S. Tsirkin wrote:
> > > > > On Wed, Jun 28, 2017 at 09:54:03AM +0800, Jason Wang wrote:
> > > > > > We should allow csumed packet for small buffer, otherwise XDP_PASS
> > > > > > won't work correctly.
> > > > > >
> > > > > > Fixes commit bb91accf2733 ("virtio-net: XDP support for small buffers")
> > > > > > Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > > > > The issue would be VIRTIO_NET_HDR_F_DATA_VALID might be set.
> > > > > What do you think?
> > > > I think it's safe. For XDP_PASS, it work like in the past.
> > > That's the part I don't get. With DATA_VALID csum in packet is wrong, XDP
> > > tools assume it's value.
> > DATA_VALID is CHECKSUM_UNCESSARY on the host, and according to the comment
> > in skbuff.h
> >
> >
> > "
> >   *   The hardware you're dealing with doesn't calculate the full checksum
> >   *   (as in CHECKSUM_COMPLETE), but it does parse headers and verify
> > checksums
> >   *   for specific protocols. For such packets it will set
> > CHECKSUM_UNNECESSARY
> >   *   if their checksums are okay. skb->csum is still undefined in this case
> >   *   though. A driver or device must never modify the checksum field in the
> >   *   packet even if checksum is verified.
> > "
> >
> > The csum is correct I believe?
> >
> > Thanks
> That's on input. But I think for tun it's output, where that is equivalent
> to CHECKSUM_NONE

Yes, but the comment said:

"
CKSUM_NONE:
 *
 *   The skb was already checksummed by the protocol, or a checksum is not
 *   required.
 *
 * CHECKSUM_UNNECESSARY:
 *
 *   This has the same meaning on as CHECKSUM_NONE for checksum offload on
 *   output.
 *
"

So still correct I think?

Thanks