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-rw-r--r--src/plugins/af_xdp/af_xdp_doc.md28
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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.