[RFC Patch v1 2/3] net: ena: update README file with a description about LPC

[Shay Agroskin <shayagr@amazon.com>]

Add a description for local page cache system to the ENA driver readme
file.

Signed-off-by: Shay Agroskin <shayagr@amazon.com>
---
 .../device_drivers/ethernet/amazon/ena.rst    | 25 +++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/Documentation/networking/device_drivers/ethernet/amazon/ena.rst b/Documentation/networking/device_drivers/ethernet/amazon/ena.rst
index 3561a8a29fd2..d3423a2f472c 100644
--- a/Documentation/networking/device_drivers/ethernet/amazon/ena.rst
+++ b/Documentation/networking/device_drivers/ethernet/amazon/ena.rst
@@ -222,6 +222,31 @@ If the frame length is larger than rx_copybreak, napi_get_frags()
 is used, otherwise netdev_alloc_skb_ip_align() is used, the buffer
 content is copied (by CPU) to the SKB, and the buffer is recycled.
 
+Local Page Cache (LPC)
+======================
+ENA Linux driver allows to reduce lock contention and improve CPU usage by
+allocating RX buffers from a page cache rather than from Linux memory system
+(PCP or buddy allocator). The cache is created and binded per RX queue, and
+pages allocated for the queue are stored in the cache (up to cache maximum
+size).
+
+When enabled, LPC cache size is ENA_LPC_DEFAULT_MULTIPLIER * 1024 (2048 by
+default) pages.
+
+The cache usage for each queue can be monitored using ``ethtool -S`` counters. Where:
+
+- *rx_queue#_lpc_warm_up* - number of pages that were allocated and stored in
+  the cache
+- *rx_queue#_lpc_full* - number of pages that were allocated without using the
+  cache because it didn't have free pages
+- *rx_queue#_lpc_wrong_numa* -  number of pages from the cache that belong to a
+  different NUMA node than the CPU which runs the NAPI routine. In this case,
+  the driver would try to allocate a new page from the same NUMA node instead
+
+LPC is disabled when using XDP or when using less than 16 queue pairs. Note that
+cache usage might increase the memory footprint of the driver (depending on the
+traffic).
+
 Statistics
 ==========
 The user can obtain ENA device and driver statistics using ethtool.
-- 
2.25.1