[net-next] hv_netvsc: Add a comment clarifying batching logic

Commit Message

Haiyang Zhang March 12, 2021, 11:45 p.m. UTC

From: Shachar Raindel <[e-mail]>

The batching logic in netvsc_send is non-trivial, due to
a combination of the Linux API and the underlying hypervisor
interface. Add a comment explaining why the code is written this
way.

Signed-off-by: Shachar Raindel <[e-mail]>
Signed-off-by: Haiyang Zhang <[e-mail]>
---
 .../ethernet/microsoft/netvsc.rst             | 14 ++++++++-----
 drivers/net/hyperv/netvsc.c                   | 20 +++++++++++++++++++
 2 files changed, 29 insertions(+), 5 deletions(-)

Comments

Dexuan Cui March 13, 2021, 4:37 a.m. UTC | #1

> From: LKML haiyangz <[e-mail]> On Behalf Of Haiyang Zhang
> Sent: Friday, March 12, 2021 3:45 PM
> To: [e-mail]; [e-mail]
> Cc: Haiyang Zhang <[e-mail]>; KY Srinivasan
> <[e-mail]>; Stephen Hemminger <[e-mail]>;
> [e-mail]; vkuznets <[e-mail]>; [e-mail];
> [e-mail]; Shachar Raindel <[e-mail]>
> Subject: [PATCH net-next] hv_netvsc: Add a comment clarifying batching logic
> 
> From: Shachar Raindel <[e-mail]>
> 
> The batching logic in netvsc_send is non-trivial, due to
> a combination of the Linux API and the underlying hypervisor
> interface. Add a comment explaining why the code is written this
> way.
> 
> Signed-off-by: Shachar Raindel <[e-mail]>
> Signed-off-by: Haiyang Zhang <[e-mail]>
> ---

Reviewed-by: Dexuan Cui <[e-mail]>

[e-mail] March 14, 2021, 9:40 p.m. UTC | #2

Hello:

This patch was applied to netdev/net-next.git (refs/heads/master):

On Fri, 12 Mar 2021 15:45:27 -0800 you wrote:
> From: Shachar Raindel <[e-mail]>
> 
> The batching logic in netvsc_send is non-trivial, due to
> a combination of the Linux API and the underlying hypervisor
> interface. Add a comment explaining why the code is written this
> way.
> 
> [...]

Here is the summary with links:
  - [net-next] hv_netvsc: Add a comment clarifying batching logic
    https://git.kernel.org/netdev/net-next/c/bd49fea7586b

You are awesome, thank you!
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Patch

diff --git a/Documentation/networking/device_drivers/ethernet/microsoft/netvsc.rst b/Documentation/networking/device_drivers/ethernet/microsoft/netvsc.rst
index c3f51c672a68..fc5acd427a5d 100644
--- a/Documentation/networking/device_drivers/ethernet/microsoft/netvsc.rst
+++ b/Documentation/networking/device_drivers/ethernet/microsoft/netvsc.rst
@@ -87,11 +87,15 @@  Receive Buffer
   contain one or more packets. The number of receive sections may be changed
   via ethtool Rx ring parameters.
 
-  There is a similar send buffer which is used to aggregate packets for sending.
-  The send area is broken into chunks of 6144 bytes, each of section may
-  contain one or more packets. The send buffer is an optimization, the driver
-  will use slower method to handle very large packets or if the send buffer
-  area is exhausted.
+  There is a similar send buffer which is used to aggregate packets
+  for sending.  The send area is broken into chunks, typically of 6144
+  bytes, each of section may contain one or more packets. Small
+  packets are usually transmitted via copy to the send buffer. However,
+  if the buffer is temporarily exhausted, or the packet to be transmitted is
+  an LSO packet, the driver will provide the host with pointers to the data
+  from the SKB. This attempts to achieve a balance between the overhead of
+  data copy and the impact of remapping VM memory to be accessible by the
+  host.
 
 XDP support
 -----------
diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c
index dc3f73c3b33e..dc333dceb055 100644
--- a/drivers/net/hyperv/netvsc.c
+++ b/drivers/net/hyperv/netvsc.c
@@ -1006,6 +1006,26 @@  static inline void move_pkt_msd(struct hv_netvsc_packet **msd_send,
 }
 
 /* RCU already held by caller */
+/* Batching/bouncing logic is designed to attempt to optimize
+ * performance.
+ *
+ * For small, non-LSO packets we copy the packet to a send buffer
+ * which is pre-registered with the Hyper-V side. This enables the
+ * hypervisor to avoid remapping the aperture to access the packet
+ * descriptor and data.
+ *
+ * If we already started using a buffer and the netdev is transmitting
+ * a burst of packets, keep on copying into the buffer until it is
+ * full or we are done collecting a burst. If there is an existing
+ * buffer with space for the RNDIS descriptor but not the packet, copy
+ * the RNDIS descriptor to the buffer, keeping the packet in place.
+ *
+ * If we do batching and send more than one packet using a single
+ * NetVSC message, free the SKBs of the packets copied, except for the
+ * last packet. This is done to streamline the handling of the case
+ * where the last packet only had the RNDIS descriptor copied to the
+ * send buffer, with the data pointers included in the NetVSC message.
+ */
 int netvsc_send(struct net_device *ndev,
 		struct hv_netvsc_packet *packet,
 		struct rndis_message *rndis_msg,