On Wed, 25 Mar 2026 09:12:38 +0000 Bruce Richardson <[email protected]> wrote:
> On Wed, Mar 25, 2026 at 08:41:39AM +0100, Morten Brørup wrote: > > > From: Stephen Hemminger [mailto:[email protected]] > > > Sent: Tuesday, 24 March 2026 18.12 > > > > > > On Mon, 16 Mar 2026 16:55:29 +0100 > > > Morten Brørup <[email protected]> wrote: > > > > > > > > > > > > > This is an example of something I previously flagged. Like with > > > real > > > > > hardware, I think the PMD should be inserting the VLAN tag into the > > > > > packet > > > > > as part of the Tx function, not the prepare function. > > > > > > > > Agree with Bruce on this. > > > > For simple stuff like VLAN offload, applications should not be > > > required to call tx_prep first. > > > > > > > > However, the Tx function is supposed to not modify the packets; > > > relevant when refcnt > 1. > > > > > > > > Instead of modifying the packet data to insert/strip the VLAN tag, > > > > perhaps the driver can split the write/read operation into multiple > > > write/read operations: > > > > 1. the Ethernet header > > > > 2. the VLAN tag > > > > 3. the remaining packet data > > > > > > > > I haven't really followed the pcap driver, so maybe my suggestion > > > doesn't make sense. > > > > > > The prepare code and VLAN was copied from virtio. > > > I assume virtio is widely used already. > > > > OK, that makes it harder to object to. > > > > Yes, but I also believe that the topic was not previously discussed and > that the virtio driver may be wrong in how it behaves. > > I still think, for consistency with HW drivers, SW drivers should do the > tagging in the Tx function. > > I also think that we should provide a DPDK-lib level helper function (be it in > ethdev or elsewhere) for doing this sort of thing for all drivers. That way > we can put in the necessary copying of packets with refcnt > 1 and have it > apply globally. > > /Bruce Virtio also requires tx_prepare if doing any form of segmentation offload. If this is not ok, it should be put somewhere in the docs and virtio fixed.
