af_xdp: workaround kernel race between poll() and sendmsg()

Prior to Linux 5.6 there is a race condition between poll() and
sendmsg() in the kernel. This patch protects the syscalls with a lock
to prevent it, unless the NO_SYSCALL_LOCK flag is set at create time.
See
https://lore.kernel.org/bpf/BYAPR11MB365382C5DB1E5FCC53242609C1549@BYAPR11MB3653.namprd11.prod.outlook.com/

Type: fix

Change-Id: Ie7d4f5cb41f697b11a09b6046e54d190430d76df
Signed-off-by: Benoît Ganne <bganne@cisco.com>

1 files changed, 27 insertions, 1 deletions

diff --git a/src/plugins/af_xdp/af_xdp_doc.md b/src/plugins/af_xdp/af_xdp_doc.md
index 7d83d712918..f5859dbb901 100644
--- a/src/plugins/af_xdp/af_xdp_doc.md
+++ b/src/plugins/af_xdp/af_xdp_doc.md
@@ -12,11 +12,15 @@ Under development: it should work, but has not been thoroughly tested.
  - custom eBPF program
  - polling, interrupt and adaptive mode
 
-## Limitations
+## Known limitations
+
+### MTU
 Because of AF_XDP restrictions, the MTU is limited to below PAGE_SIZE
 (4096-bytes on most systems) minus 256-bytes, and they are additional
 limitations depending upon specific Linux device drivers.
 As a rule of thumb, a MTU of 3000-bytes or less should be safe.
+
+### Number of buffers
 Furthermore, upon UMEM creation, the kernel allocates a
 physically-contiguous structure, whose size is proportional to the number
 of 4KB pages contained in the UMEM. That allocation might fail when
@@ -25,7 +29,29 @@ controlled with the `buffers { buffers-per-numa }` configuration option.
 Finally, note that because of this limitation, this plugin is unlikely
 to be compatible with the use of 1GB hugepages.
 
+### Interrupt mode
+Interrupt and adaptive mode are supported but is limited by default to single
+threaded (no worker) configurations because of a kernel limitation prior to
+5.6. You can bypass the limitation at interface creation time by adding the
+`no-syscall-lock` parameter, but you must be sure that your kernel can
+support it, otherwise you will experience double-frees.
+See
+https://lore.kernel.org/bpf/BYAPR11MB365382C5DB1E5FCC53242609C1549@BYAPR11MB3653.namprd11.prod.outlook.com/
+for more details.
+
+### Mellanox
+When setting the number of queues on Mellanox NIC with `ethtool -L`, you must
+use twice the amount of configured queues: it looks like the Linux driver will
+create separate RX queues and TX queues (but all queues can be used for both
+RX and TX, the NIC will just not sent any packet on "pure" TX queues.
+Confused? So I am.). For example if you set `combined 2` you will effectively
+have to create 4 rx queues in AF_XDP if you want to be sure to receive all
+packets.
+
 ## Requirements
+This drivers supports Linux kernel 5.4 and later. Kernels older than 5.4 are
+missing unaligned buffers support.
+
 The Linux kernel interface must be up and have enough queues before
 creating the VPP AF_XDP interface, otherwise Linux will deny creating
 the AF_XDP socket.