Subject: Re: [External] Re: [PATCH v2] virtio_net: Fix mismatched buf address when unmapping for small packets

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 4 Sep 2024 06:57:30 -0400

On Wed, Sep 04, 2024 at 03:30:02PM +0800, Xuan Zhuo wrote:
> On Wed, 4 Sep 2024 15:21:28 +0800, =?utf-8?b?5paH5Y2a5p2O?= <[email-masked]> wrote:
> > When SWIOTLB is enabled, a DMA map will allocate a bounce buffer for real
> > DMA operations,
> > and when unmapping, SWIOTLB copies the content in the bounce buffer to the
> > driver-allocated
> > buffer (the `buf` variable). Such copy only synchronizes data in the buffer
> > range, not the whole page.
> > So we should give the correct offset for DMA unmapping.
> 
> I see.
> 
> But I think we should pass the "correct" buf to virtio core as the "data" by
> virtqueue_add_inbuf_ctx().
> 
> In the merge mode, we pass the pointer that points to the virtnet header.
> In the small mode, we pass the pointer that points to the virtnet header - offset.
> 
> But this is not the only problem, we try to get the virtnet header by the buf
> inside receive_buf(before receive_small).
> 
> 	flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
> 
> So I think it is time to unify the buf that passed to the virtio core into a
> pointer pointed to the virtnet header.
> 
> Thanks.

Hard to say without seeing what is proposed, but I think
data pointer is opaque to virtio, it does not have to
point to anything specific - just !NULL.

> 
> >
> > Thanks.
> >
> > On Wed, Sep 4, 2024 at 2:46 PM Xuan Zhuo <[email-masked]> wrote:
> > >
> > > On Wed,  4 Sep 2024 14:10:09 +0800, Wenbo Li <[email-masked]>
> > wrote:
> > > > Currently, the virtio-net driver will perform a pre-dma-mapping for
> > > > small or mergeable RX buffer. But for small packets, a mismatched
> > address
> > > > without VIRTNET_RX_PAD and xdp_headroom is used for unmapping.
> > >
> > > Will used virt_to_head_page(), so could you say more about it?
> > >
> > >         struct page *page = virt_to_head_page(buf);
> > >
> > > Thanks.
> > >
> > > >
> > > > That will result in unsynchronized buffers when SWIOTLB is enabled, for
> > > > example, when running as a TDX guest.
> > > >
> > > > This patch handles small and mergeable packets separately and fixes
> > > > the mismatched buffer address.
> > > >
> > > > Changes from v1: Use ctx to get xdp_headroom.
> > > >
> > > > Fixes: 295525e29a5b ("virtio_net: merge dma operations when filling
> > mergeable buffers")
> > > > Signed-off-by: Wenbo Li <[email-masked]>
> > > > Signed-off-by: Jiahui Cen <[email-masked]>
> > > > Signed-off-by: Ying Fang <[email-masked]>
> > > > ---
> > > >  drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++++++-
> > > >  1 file changed, 28 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > > index c6af18948..cbc3c0ae4 100644
> > > > --- a/drivers/net/virtio_net.c
> > > > +++ b/drivers/net/virtio_net.c
> > > > @@ -891,6 +891,23 @@ static void *virtnet_rq_get_buf(struct
> > receive_queue *rq, u32 *len, void **ctx)
> > > >       return buf;
> > > >  }
> > > >
> > > > +static void *virtnet_rq_get_buf_small(struct receive_queue *rq,
> > > > +                                   u32 *len,
> > > > +                                   void **ctx,
> > > > +                                   unsigned int header_offset)
> > > > +{
> > > > +     void *buf;
> > > > +     unsigned int xdp_headroom;
> > > > +
> > > > +     buf = virtqueue_get_buf_ctx(rq->vq, len, ctx);
> > > > +     if (buf) {
> > > > +             xdp_headroom = (unsigned long)*ctx;
> > > > +             virtnet_rq_unmap(rq, buf + VIRTNET_RX_PAD + xdp_headroom,
> > *len);
> > > > +     }
> > > > +
> > > > +     return buf;
> > > > +}
> > > > +
> > > >  static void virtnet_rq_init_one_sg(struct receive_queue *rq, void
> > *buf, u32 len)
> > > >  {
> > > >       struct virtnet_rq_dma *dma;
> > > > @@ -2692,13 +2709,23 @@ static int virtnet_receive_packets(struct
> > virtnet_info *vi,
> > > >       int packets = 0;
> > > >       void *buf;
> > > >
> > > > -     if (!vi->big_packets || vi->mergeable_rx_bufs) {
> > > > +     if (vi->mergeable_rx_bufs) {
> > > >               void *ctx;
> > > >               while (packets < budget &&
> > > >                      (buf = virtnet_rq_get_buf(rq, &len, &ctx))) {
> > > >                       receive_buf(vi, rq, buf, len, ctx, xdp_xmit,
> > stats);
> > > >                       packets++;
> > > >               }
> > > > +     } else if (!vi->big_packets) {
> > > > +             void *ctx;
> > > > +             unsigned int xdp_headroom = virtnet_get_headroom(vi);
> > > > +             unsigned int header_offset = VIRTNET_RX_PAD +
> > xdp_headroom;
> > > > +
> > > > +             while (packets < budget &&
> > > > +                    (buf = virtnet_rq_get_buf_small(rq, &len, &ctx,
> > header_offset))) {
> > > > +                     receive_buf(vi, rq, buf, len, ctx, xdp_xmit,
> > stats);
> > > > +                     packets++;
> > > > +             }
> > > >       } else {
> > > >               while (packets < budget &&
> > > >                      (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
> > > > --
> > > > 2.20.1
> > > >
> >