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Date: Mon, 5 Dec 2016 11:55:19 -0800
From: Martin KaFai Lau <kafai@...com>
To: Saeed Mahameed <saeedm@....mellanox.co.il>
CC: Linux Netdev List <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...com>,
Brenden Blanco <bblanco@...mgrid.com>,
Daniel Borkmann <daniel@...earbox.net>,
David Miller <davem@...emloft.net>,
"Jesper Dangaard Brouer" <brouer@...hat.com>,
Saeed Mahameed <saeedm@...lanox.com>,
"Tariq Toukan" <tariqt@...lanox.com>,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 net-next 3/4] mlx4: xdp: Reserve headroom for
receiving packet when XDP prog is active
On Mon, Dec 05, 2016 at 02:54:06AM +0200, Saeed Mahameed wrote:
> On Sun, Dec 4, 2016 at 5:17 AM, Martin KaFai Lau <kafai@...com> wrote:
> > Reserve XDP_PACKET_HEADROOM and honor bpf_xdp_adjust_head()
> > when XDP prog is active. This patch only affects the code
> > path when XDP is active.
> >
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > ---
>
> Hi Martin, Sorry for the late review, i have some comments below
>
> > drivers/net/ethernet/mellanox/mlx4/en_netdev.c | 17 +++++++++++++++--
> > drivers/net/ethernet/mellanox/mlx4/en_rx.c | 23 +++++++++++++++++------
> > drivers/net/ethernet/mellanox/mlx4/en_tx.c | 9 +++++----
> > drivers/net/ethernet/mellanox/mlx4/mlx4_en.h | 3 ++-
> > 4 files changed, 39 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
> > index 311c14153b8b..094a13b52cf6 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
> > +++ b/drivers/net/ethernet/mellanox/mlx4/en_netdev.c
> > @@ -51,7 +51,8 @@
> > #include "mlx4_en.h"
> > #include "en_port.h"
> >
> > -#define MLX4_EN_MAX_XDP_MTU ((int)(PAGE_SIZE - ETH_HLEN - (2 * VLAN_HLEN)))
> > +#define MLX4_EN_MAX_XDP_MTU ((int)(PAGE_SIZE - ETH_HLEN - (2 * VLAN_HLEN) - \
> > + XDP_PACKET_HEADROOM))
> >
> > int mlx4_en_setup_tc(struct net_device *dev, u8 up)
> > {
> > @@ -1551,6 +1552,7 @@ int mlx4_en_start_port(struct net_device *dev)
> > struct mlx4_en_tx_ring *tx_ring;
> > int rx_index = 0;
> > int err = 0;
> > + int mtu;
> > int i, t;
> > int j;
> > u8 mc_list[16] = {0};
> > @@ -1684,8 +1686,12 @@ int mlx4_en_start_port(struct net_device *dev)
> > }
> >
> > /* Configure port */
> > + mtu = priv->rx_skb_size + ETH_FCS_LEN;
> > + if (priv->tx_ring_num[TX_XDP])
> > + mtu += XDP_PACKET_HEADROOM;
> > +
>
> Why would the physical MTU care for the headroom you preserve for XDP prog?
> This is the wire MTU, it shouldn't be changed, please keep it as
> before, any preservation you make in packets buffers are needed only
> for FWD case or modify case (HW or wire should not care about them).
Thanks for your feedback!
FWD:
packet received from a port
=> process by a XDP prog
=> XDP_TX out to the same port.
For example, if the received packet has 1500 payload and the XDP prog
encapsulates it in an IPv6 header (+40 bytes). After testing, it cannot
be sent out due to the HW/wire MTU is 1500.
Even the wire MTU info was passed to the XDP prog, there is not much a
XDP prog could do here other than dropping it.
Hence, this patch gives guarantee to the XDP prog such that
it can always send out what it has received + XDP_PACKET_HEADROOM.
>
> > err = mlx4_SET_PORT_general(mdev->dev, priv->port,
> > - priv->rx_skb_size + ETH_FCS_LEN,
> > + mtu,
> > priv->prof->tx_pause,
> > priv->prof->tx_ppp,
> > priv->prof->rx_pause,
> > @@ -2255,6 +2261,13 @@ static bool mlx4_en_check_xdp_mtu(struct net_device *dev, int mtu)
> > {
> > struct mlx4_en_priv *priv = netdev_priv(dev);
> >
> > + if (mtu + XDP_PACKET_HEADROOM > priv->max_mtu) {
> > + en_err(priv,
> > + "Device max mtu:%d does not allow %d bytes reserved headroom for XDP prog\n",
> > + priv->max_mtu, XDP_PACKET_HEADROOM);
> > + return false;
> > + }
> > +
> > if (mtu > MLX4_EN_MAX_XDP_MTU) {
> > en_err(priv, "mtu:%d > max:%d when XDP prog is attached\n",
> > mtu, MLX4_EN_MAX_XDP_MTU);
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_rx.c b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > index 23e9d04d1ef4..324771ac929e 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx4/en_rx.c
> > @@ -96,7 +96,6 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
> > struct mlx4_en_rx_alloc page_alloc[MLX4_EN_MAX_RX_FRAGS];
> > const struct mlx4_en_frag_info *frag_info;
> > struct page *page;
> > - dma_addr_t dma;
> > int i;
> >
> > for (i = 0; i < priv->num_frags; i++) {
> > @@ -115,9 +114,10 @@ static int mlx4_en_alloc_frags(struct mlx4_en_priv *priv,
> >
> > for (i = 0; i < priv->num_frags; i++) {
> > frags[i] = ring_alloc[i];
> > - dma = ring_alloc[i].dma + ring_alloc[i].page_offset;
> > + frags[i].page_offset += priv->frag_info[i].rx_headroom;
>
> I don't see any need for headroom on frag_info other that frag0 (which
> where the packet starts).
> What is the meaning of a headroom of a frag in a middle of a packet ?
>
> if you agree with me then, you can use XDP_PACKET_HEADROOM as is where
> needed (i.e frag0 page offset) and remove
> "priv->frag_info[i].rx_headroom"
>
> ...
>
> After going through the code a little bit i see that this code is
> shared between XDP and common path, and you didn't want to add boolean
> conditions.
>
> Ok i see what you did here.
>
> Maybe we can pass headroom as a function parameter and split frag0
> handling from the rest ?
> If it is too much then i am ok with the code as it is,
Right, this patch does the boolean check (XDP active or not) early on
in mlx4_en_calc_rx_buf() (i.e. out of the fast path) and store
the result in priv->frag_info[0].rx_headroom.
Just want to ensure I understand your comment correctly.
You prefer not to store the boolean test result in frag_info[0].rx_headroom
since it is redundant to !!priv->tx_ring_num[TX_XDP] and rx_headroom is also
confusing for frag[1-3].
Instead, do the XDP [in]active test before calling mlx4_en_alloc_frags()
and then only adjust frags[0].page_offset by +XDP_PACKET_HEADROOM if is needed.
It could be done either by passing an extra argument to mlx4_en_alloc_frags()
or completely separate mlx4_en_alloc_frags(). I am fine with this also.
>
> > + rx_desc->data[i].addr = cpu_to_be64(frags[i].dma +
> > + frags[i].page_offset);
> > ring_alloc[i] = page_alloc[i];
> > - rx_desc->data[i].addr = cpu_to_be64(dma);
> > }
> >
> > return 0;
> > @@ -250,7 +250,8 @@ static int mlx4_en_prepare_rx_desc(struct mlx4_en_priv *priv,
> >
> > if (ring->page_cache.index > 0) {
> > frags[0] = ring->page_cache.buf[--ring->page_cache.index];
> > - rx_desc->data[0].addr = cpu_to_be64(frags[0].dma);
> > + rx_desc->data[0].addr = cpu_to_be64(frags[0].dma +
> > + frags[0].page_offset);
> > return 0;
> > }
> >
> > @@ -889,6 +890,7 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
> > if (xdp_prog) {
> > struct xdp_buff xdp;
> > dma_addr_t dma;
> > + void *pg_addr, *orig_data;
> > u32 act;
> >
> > dma = be64_to_cpu(rx_desc->data[0].addr);
> > @@ -896,11 +898,18 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
> > priv->frag_info[0].frag_size,
> > DMA_FROM_DEVICE);
> >
> > - xdp.data = page_address(frags[0].page) +
> > - frags[0].page_offset;
> > + pg_addr = page_address(frags[0].page);
> > + orig_data = pg_addr + frags[0].page_offset;
> > + xdp.data = orig_data;
> > xdp.data_end = xdp.data + length;
> >
> > act = bpf_prog_run_xdp(xdp_prog, &xdp);
> > +
> > + if (xdp.data != orig_data) {
> > + length = xdp.data_end - xdp.data;
> > + frags[0].page_offset = xdp.data - pg_addr;
> > + }
> > +
> >
>
> is this needed only for XDP FWD case ?
No. It is also for PASS.
> is this the only way to detect that the user modified the packet
> headers (comparing pointers, before and after) ?
Yes
>
> if the answer is yes, it should be faster to unconditionally reset
> packet offset and lenght on XDP_FWD :
> case XDP_FWD:
> length = xdp.data_end - xdp.data;
> frags[0].page_offset = xdp.data - pg_addr;
>
>
> > switch (act) {
> > case XDP_PASS:
> > break;
> > @@ -1180,6 +1189,7 @@ void mlx4_en_calc_rx_buf(struct net_device *dev)
> > */
> > priv->frag_info[0].frag_stride = PAGE_SIZE;
> > priv->frag_info[0].dma_dir = PCI_DMA_BIDIRECTIONAL;
> > + priv->frag_info[0].rx_headroom = XDP_PACKET_HEADROOM;
> > i = 1;
> > } else {
> > int buf_size = 0;
> > @@ -1194,6 +1204,7 @@ void mlx4_en_calc_rx_buf(struct net_device *dev)
> > ALIGN(priv->frag_info[i].frag_size,
> > SMP_CACHE_BYTES);
> > priv->frag_info[i].dma_dir = PCI_DMA_FROMDEVICE;
> > + priv->frag_info[i].rx_headroom = 0;
>
> IMHO, redundant. as you see here frag0 and other frags handling are
> separated, maybe we can do the same in mlx4_en_alloc_frags.
>
> > buf_size += priv->frag_info[i].frag_size;
> > i++;
> > }
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> > index 4b597dca5c52..9e5f38cefe5f 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> > +++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
> > @@ -354,7 +354,7 @@ u32 mlx4_en_recycle_tx_desc(struct mlx4_en_priv *priv,
> > struct mlx4_en_rx_alloc frame = {
> > .page = tx_info->page,
> > .dma = tx_info->map0_dma,
> > - .page_offset = 0,
> > + .page_offset = XDP_PACKET_HEADROOM,
> > .page_size = PAGE_SIZE,
> > };
> >
> > @@ -1132,7 +1132,7 @@ netdev_tx_t mlx4_en_xmit_frame(struct mlx4_en_rx_ring *rx_ring,
> > tx_info->page = frame->page;
> > frame->page = NULL;
> > tx_info->map0_dma = dma;
> > - tx_info->map0_byte_count = length;
> > + tx_info->map0_byte_count = length + frame->page_offset;
>
> Didn't you already take care of lenght by the following code:
> if (xdp.data != orig_data) {
> length = xdp.data_end - xdp.data;
> frags[0].page_offset = xdp.data - pg_addr;
> }
>
Before this patch, length always assumes the data starts at the beginning
of the page and dma is the start of the page. Hence, adding
framg->page_offset back to the length here.
However, if I read the codes correctly, I think the map0_byte_count (before or
after this patch) does not matter since it is only used in dma_unmap_page() and
PAGE_SIZE is always used in dma_unmap_page() for this code patch. Hence, I think
we can just set map0_byte_count to PAGE_SIZE here.
> and here frame->page_offset is not really page offset, it can only be
> XDP_PACKET_HEADROOM.
Note that the XDP prog can call bpf_xdp_adjust_head() to add a header.
The XDP prog can extend up to XDP_PACKET_HEADROOM (256) bytes but it
can also (and usually) only add 40 bytes IPv6 header and then XDP_TX it out.
>
> > tx_info->nr_txbb = nr_txbb;
> > tx_info->nr_bytes = max_t(unsigned int, length, ETH_ZLEN);
> > tx_info->data_offset = (void *)data - (void *)tx_desc;
> > @@ -1141,9 +1141,10 @@ netdev_tx_t mlx4_en_xmit_frame(struct mlx4_en_rx_ring *rx_ring,
> > tx_info->linear = 1;
> > tx_info->inl = 0;
> >
> > - dma_sync_single_for_device(priv->ddev, dma, length, PCI_DMA_TODEVICE);
> > + dma_sync_single_range_for_device(priv->ddev, dma, frame->page_offset,
> > + length, PCI_DMA_TODEVICE);
> >
> > - data->addr = cpu_to_be64(dma);
> > + data->addr = cpu_to_be64(dma + frame->page_offset);
> > data->lkey = ring->mr_key;
> > dma_wmb();
> > data->byte_count = cpu_to_be32(length);
> > diff --git a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> > index 20a936428f4a..ba1c6cd0cc79 100644
> > --- a/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> > +++ b/drivers/net/ethernet/mellanox/mlx4/mlx4_en.h
> > @@ -475,7 +475,8 @@ struct mlx4_en_frag_info {
> > u16 frag_prefix_size;
> > u32 frag_stride;
> > enum dma_data_direction dma_dir;
> > - int order;
> > + u16 order;
> > + u16 rx_headroom;
> > };
> >
> > #ifdef CONFIG_MLX4_EN_DCB
> > --
> > 2.5.1
> >
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