Subject: RE: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support

From: wangyunjian <[email-masked]>
Date: Mon, 11 Mar 2024 13:27:54 +0000

> -----Original Message-----
> From: Jason Wang [mailto:[email-masked]]
> Sent: Monday, March 11, 2024 12:01 PM
> To: wangyunjian <[email-masked]>
> Cc: Michael S. Tsirkin <[email-masked]>; Paolo Abeni <[email-masked]>;
> [email-masked]; [email-masked]; [email-masked];
> [email-masked]; [email-masked];
> [email-masked]; [email-masked]; [email-masked];
> [email-masked]; [email-masked]; [email-masked];
> [email-masked]; xudingke <[email-masked]>; liwei (DT)
> <[email-masked]>
> Subject: Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy support
> 
> On Mon, Mar 4, 2024 at 9:45 PM wangyunjian <[email-masked]>
> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Michael S. Tsirkin [mailto:[email-masked]]
> > > Sent: Friday, March 1, 2024 7:53 PM
> > > To: wangyunjian <[email-masked]>
> > > Cc: Paolo Abeni <[email-masked]>;
> > > [email-masked]; [email-masked];
> > > [email-masked]; [email-masked]; [email-masked];
> > > [email-masked]; [email-masked];
> > > [email-masked]; [email-masked]; [email-masked];
> > > [email-masked]; [email-masked];
> > > [email-masked]; xudingke <[email-masked]>;
> > > liwei (DT) <[email-masked]>
> > > Subject: Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy
> > > support
> > >
> > > On Fri, Mar 01, 2024 at 11:45:52AM +0000, wangyunjian wrote:
> > > > > -----Original Message-----
> > > > > From: Paolo Abeni [mailto:[email-masked]]
> > > > > Sent: Thursday, February 29, 2024 7:13 PM
> > > > > To: wangyunjian <[email-masked]>; [email-masked];
> > > > > [email-masked]; [email-masked];
> > > > > [email-masked]; [email-masked]; [email-masked];
> > > > > [email-masked]; [email-masked];
> > > > > [email-masked]
> > > > > Cc: [email-masked]; [email-masked];
> > > > > [email-masked]; [email-masked];
> > > > > [email-masked]; xudingke <[email-masked]>;
> > > > > liwei (DT) <[email-masked]>
> > > > > Subject: Re: [PATCH net-next v2 3/3] tun: AF_XDP Tx zero-copy
> > > > > support
> > > > >
> > > > > On Wed, 2024-02-28 at 19:05 +0800, Yunjian Wang wrote:
> > > > > > @@ -2661,6 +2776,54 @@ static int tun_ptr_peek_len(void *ptr)
> > > > > >         }
> > > > > >  }
> > > > > >
> > > > > > +static void tun_peek_xsk(struct tun_file *tfile) {
> > > > > > +       struct xsk_buff_pool *pool;
> > > > > > +       u32 i, batch, budget;
> > > > > > +       void *frame;
> > > > > > +
> > > > > > +       if (!ptr_ring_empty(&tfile->tx_ring))
> > > > > > +               return;
> > > > > > +
> > > > > > +       spin_lock(&tfile->pool_lock);
> > > > > > +       pool = tfile->xsk_pool;
> > > > > > +       if (!pool) {
> > > > > > +               spin_unlock(&tfile->pool_lock);
> > > > > > +               return;
> > > > > > +       }
> > > > > > +
> > > > > > +       if (tfile->nb_descs) {
> > > > > > +               xsk_tx_completed(pool, tfile->nb_descs);
> > > > > > +               if (xsk_uses_need_wakeup(pool))
> > > > > > +                       xsk_set_tx_need_wakeup(pool);
> > > > > > +       }
> > > > > > +
> > > > > > +       spin_lock(&tfile->tx_ring.producer_lock);
> > > > > > +       budget = min_t(u32, tfile->tx_ring.size,
> > > > > > + TUN_XDP_BATCH);
> > > > > > +
> > > > > > +       batch = xsk_tx_peek_release_desc_batch(pool, budget);
> > > > > > +       if (!batch) {
> > > > >
> > > > > This branch looks like an unneeded "optimization". The generic
> > > > > loop below should have the same effect with no measurable perf
> > > > > delta - and
> > > smaller code.
> > > > > Just remove this.
> > > > >
> > > > > > +               tfile->nb_descs = 0;
> > > > > > +               spin_unlock(&tfile->tx_ring.producer_lock);
> > > > > > +               spin_unlock(&tfile->pool_lock);
> > > > > > +               return;
> > > > > > +       }
> > > > > > +
> > > > > > +       tfile->nb_descs = batch;
> > > > > > +       for (i = 0; i < batch; i++) {
> > > > > > +               /* Encode the XDP DESC flag into lowest bit
> > > > > > + for consumer to
> > > differ
> > > > > > +                * XDP desc from XDP buffer and sk_buff.
> > > > > > +                */
> > > > > > +               frame = tun_xdp_desc_to_ptr(&pool->tx_descs[i]);
> > > > > > +               /* The budget must be less than or equal to
> tx_ring.size,
> > > > > > +                * so enqueuing will not fail.
> > > > > > +                */
> > > > > > +               __ptr_ring_produce(&tfile->tx_ring, frame);
> > > > > > +       }
> > > > > > +       spin_unlock(&tfile->tx_ring.producer_lock);
> > > > > > +       spin_unlock(&tfile->pool_lock);
> > > > >
> > > > > More related to the general design: it looks wrong. What if
> > > > > get_rx_bufs() will fail (ENOBUF) after successful peeking? With
> > > > > no more incoming packets, later peek will return 0 and it looks
> > > > > like that the half-processed packets will stay in the ring forever???
> > > > >
> > > > > I think the 'ring produce' part should be moved into tun_do_read().
> > > >
> > > > Currently, the vhost-net obtains a batch descriptors/sk_buffs from
> > > > the ptr_ring and enqueue the batch descriptors/sk_buffs to the
> > > > virtqueue'queue, and then consumes the descriptors/sk_buffs from
> > > > the virtqueue'queue in sequence. As a result, TUN does not know
> > > > whether the batch descriptors have been used up, and thus does not
> > > > know when to
> > > return the batch descriptors.
> > > >
> > > > So, I think it's reasonable that when vhost-net checks ptr_ring is
> > > > empty, it calls peek_len to get new xsk's descs and return the descriptors.
> > > >
> > > > Thanks
> > >
> > > What you need to think about is that if you peek, another call in
> > > parallel can get the same value at the same time.
> >
> > Thank you. I have identified a problem. The tx_descs array was created within
> xsk's pool.
> > When xsk is freed, the pool and tx_descs are also freed. Howerver,
> > some descs may remain in the virtqueue'queue, which could lead to a
> use-after-free scenario.
> 
> This can probably solving by when xsk pool is disabled, signal the vhost_net to
> drop those descriptors.

I think TUN can notify vhost_net to drop these descriptors through netdev events.
However, there is a potential concurrency problem. When handling netdev events
and packets, vhost_net preempts the 'vq->mutex_lock', leading to unstable performance.

Thanks
> 
> Thanks
> 
> > Currently,
> > I do not have an idea to solve this concurrency problem and believe
> > this scenario may not be appropriate for reusing the ptr_ring.
> >
> > Thanks
> >
> > >
> > >
> > > > >
> > > > > Cheers,
> > > > >
> > > > > Paolo
> > > >
> >