doc: add instructions for WC in ENAv2

ENAv2 hardware provides Low Latency Queue v2 (LLQv2). It needs kernel
PCI driver to support write combining (WC). This patch add information
how to use it with igb_uio and vfio-pci drivers.

Signed-off-by: Rafal Kozik <rk@semihalf.com>
Acked-by: Michal Krawczyk <mk@semihalf.com>

diff --git a/doc/guides/nics/ena.rst b/doc/guides/nics/ena.rst
index 9f0a650..80da4b6 100644 (file)
--- a/doc/guides/nics/ena.rst
+++ b/doc/guides/nics/ena.rst
@@ -183,10 +183,22 @@ Prerequisites
 #. Prepare the system as recommended by DPDK suite.  This includes environment
    variables, hugepages configuration, tool-chains and configuration.
 
-#. ENA PMD can operate with ``vfio-pci`` or ``igb_uio`` driver.
+#. ENA PMD can operate with ``vfio-pci``(*) or ``igb_uio`` driver.
+
+   (*) ENAv2 hardware supports Low Latency Queue v2 (LLQv2). This feature
+   reduces the latency of the packets by pushing the header directly through
+   the PCI to the device, before the DMA is even triggered. For proper work
+   kernel PCI driver must support write combining (WC). In mainline version of
+   ``igb_uio`` (in DPDK repo) it must be enabled by loding module with
+   ``wc_activate=1`` flag (example below). However, mainline's vfio-pci
+   driver in kernel doesn't have WC support yet (planed to be added).
+   If vfio-pci used user should be either turn off ENAv2 (to avoid performance
+   impact) or recompile vfio-pci driver with patch provided in
+   `amzn-github <https://github.com/amzn/amzn-drivers/tree/master/userspace/dpdk/enav2-vfio-patch>`_.
 
 #. Insert ``vfio-pci`` or ``igb_uio`` kernel module using the command
-   ``modprobe vfio-pci`` or ``modprobe igb_uio`` respectively.
+   ``modprobe vfio-pci`` or ``modprobe uio; insmod igb_uio.ko wc_activate=1``
+   respectively.
 
 #. For ``vfio-pci`` users only:
    Please make sure that ``IOMMU`` is enabled in your system,