On Fri, Apr 4, 2025 at 10:24 PM Jon Kohler <j...@nutanix.com> wrote:
>
> Commit 098eadce3c62 ("vhost_net: disable zerocopy by default") disabled
> the module parameter for the handle_tx_zerocopy path back in 2019,
> nothing that many downstream distributions (e.g., RHEL7 and later) had
> already done the same.
>
> Both upstream and downstream disablement suggest this path is rarely
> used.
>
> Testing the module parameter shows that while the path allows packet
> forwarding, the zerocopy functionality itself is broken. On outbound
> traffic (guest TX -> external), zerocopy SKBs are orphaned by either
> skb_orphan_frags_rx() (used with the tun driver via tun_net_xmit())This is by design to avoid DOS. > or > skb_orphan_frags() elsewhere in the stack, Basically zerocopy is expected to work for guest -> remote case, so could we still hit skb_orphan_frags() in this case? > as vhost_net does not set > SKBFL_DONT_ORPHAN. > > Orphaning enforces a memcpy and triggers the completion callback, which > increments the failed TX counter, effectively disabling zerocopy again. > > Even after addressing these issues to prevent SKB orphaning and error > counter increments, performance remains poor. By default, only 64 > messages can be zerocopied, which is immediately exhausted by workloads > like iperf, resulting in most messages being memcpy'd anyhow. > > Additionally, memcpy'd messages do not benefit from the XDP batching > optimizations present in the handle_tx_copy path. > > Given these limitations and the lack of any tangible benefits, remove > zerocopy entirely to simplify the code base. > > Signed-off-by: Jon Kohler <j...@nutanix.com> Any chance we can fix those issues? Actually, we had a plan to make use of vhost-net and its tx zerocopy (or even implement the rx zerocopy) in pasta. Eugenio may explain more here. Thanks
