Re: RE: [PATCH v7 bpf-next 6/6] xsk: build skb by page (aka generic zerocopy xmit)

Xuan Zhuo wrote:

On Wed, 17 Feb 2021 16:46:04 -0800, John Fastabend [off-list ref] wrote:

quoted

Alexander Lobakin wrote:

quoted

From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

This patch is used to construct skb based on page to save memory copy
overhead.

This function is implemented based on IFF_TX_SKB_NO_LINEAR. Only the
network card priv_flags supports IFF_TX_SKB_NO_LINEAR will use page to
directly construct skb. If this feature is not supported, it is still
necessary to copy data to construct skb.

---------------- Performance Testing ------------

The test environment is Aliyun ECS server.
Test cmd:

xdpsock -i eth0 -t  -S -s <msg size>

Test result data:

size    64      512     1024    1500
copy    1916747 1775988 1600203 1440054
page    1974058 1953655 1945463 1904478
percent 3.0%    10.0%   21.58%  32.3%

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
[ alobakin:
 - expand subject to make it clearer;
 - improve skb->truesize calculation;
 - reserve some headroom in skb for drivers;
 - tailroom is not needed as skb is non-linear ]
Signed-off-by: Alexander Lobakin <redacted>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
---

[...]

quoted

+	buffer = xsk_buff_raw_get_data(pool, addr);
+	offset = offset_in_page(buffer);
+	addr = buffer - pool->addrs;
+
+	for (copied = 0, i = 0; copied < len; i++) {
+		page = pool->umem->pgs[addr >> PAGE_SHIFT];

Looks like we could walk off the end of pgs[] if len is larger than
the number of pgs? Do we need to guard against a misconfigured socket
causing a panic here? AFAIU len here is read from the user space
descriptor so is under user control. Or maybe I missed a check somewhere.

Thanks,
John

Don't worry about this, the legality of desc has been checked.

xskq_cons_peek_desc -> xskq_cons_read_desc ->
                   xskq_cons_is_valid_desc -> xp_validate_desc

Ah OK I didn't dig past the cons_read_desc(). In that case LGTM.

Acked-by: John Fastabend <john.fastabend@gmail.com>
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization