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Subject: Re: [virtio-comment] Re: [PATCH] virtio-net: Add equal-sized receive buffers feature


On 2019/11/1 äå12:09, Michael S. Tsirkin wrote:

On Thu, Oct 31, 2019 at 10:46:55AM +0000, Vitaly Mireyno wrote:

The feature is limited to receive buffers only.
A driver decides on receive buffer length. The only limitation is that this length has to be the same for all receive virtqueue buffers.
The driver configures receive buffer length to the device during device initialization, and the device reads it and may use it for optimal operation.
No changes for transmit buffers.

-----Original Message-----
From: virtio-comment@lists.oasis-open.org <virtio-comment@lists.oasis-open.org> On Behalf Of Jason Wang
Sent: Thursday, 31 October, 2019 12:15
To: Vitaly Mireyno <vmireyno@marvell.com>; virtio-comment@lists.oasis-open.org
Cc: Michael S. Tsirkin <mst@redhat.com>
Subject: [virtio-comment] Re: [PATCH] virtio-net: Add equal-sized receive buffers feature


On 2019/10/31 äå5:23, Vitaly Mireyno wrote:

Some devices benefit from working with receive buffers of a predefined constant length.
Add a feature for drivers that allocate equal-sized receive buffers, and devices that benefit from predefined receive buffers length.

Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
---
   content.tex | 29 +++++++++++++++++++++++++++--
   1 file changed, 27 insertions(+), 2 deletions(-)


Is there any other networking device that has this feature?



diff --git a/content.tex b/content.tex index b1ea9b9..c9e67c8 100644
--- a/content.tex
+++ b/content.tex
@@ -2811,6 +2811,10 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
   \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
       channel.
+\item[VIRTIO_NET_F_CONST_RXBUF_LEN(58)] Driver allocates all receive buffers 
+    of the same constant length. Device benefits from working with
+    receive buffers of equal length.
+

Problem is, I don't think linux will use this for skbs since it breaks
buffer accounting. This is because it is important to make skbs
as small as you can. So even if you see "device would benefit"
there is no way to balance this with the benefit to linux.
How do you know which benefit is bigger?



I guess the idea is e.g for Linux driver, it can refuse this feature.




You also never explained how does device benefit. My guess is this
allows device to calculate the # of descriptors to fetch
that are needed for a packet. Right?
Assuming this, I think a rough estimate should be enough.
If device fetches too much it can discard extra, if it does not
fetch enough it can fetch extra.

Let us not forget that express packets are 256 bytes so
up to 16 descriptors fit in a packet, there is no
benefit most of the time in knowing whether e.g. 1 or 2
descriptors are needed.


Let us not forget these are buffers, not descriptors.



I guess maybe the initial motivation is constant descriptor length.




So device does not really know the exact # of descriptors:
current drivers do 1 descriptor per buffer but nothing
prevents more.


Thoughts?



   \item[VIRTIO_NET_F_RSC_EXT(61)] Device can process duplicated ACKs
       and report number of coalesced segments and duplicated ACKs
@@ -2854,8 +2858,8 @@ \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network 
   \subsection{Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout}
   \label{sec:Device Types / Block Device / Feature bits / Device
configuration layout}
-Three driver-read-only configuration fields are currently defined. 
The \field{mac} address field -always exists (though is only valid if
VIRTIO_NET_F_MAC is set), and
+The driver-read-only \field{mac} address field always exists (though
+is only valid if VIRTIO_NET_F_MAC is set), and
   \field{status} only exists if VIRTIO_NET_F_STATUS is set. Two
   read-only bits (for the driver) are currently defined for the status field:
   VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.
@@ -2875,12 +2879,17 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
   VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the driver to
   use.
+The device-read-only field \field{rx_buf_len} only exists if 

Should be driver-read-only.



+VIRTIO_NET_F_CONST_RXBUF_LEN is negotiated. This field specifies the
+receive buffers length.
+
   \begin{lstlisting}
   struct virtio_net_config {
           u8 mac[6];
           le16 status;
           le16 max_virtqueue_pairs;
           le16 mtu;
+        le32 rx_buf_len;
   };
   \end{lstlisting}
@@ -2933,6 +2942,13 @@ \subsection{Device configuration 
layout}\label{sec:Device Types / Network Device
A driver SHOULD negotiate the VIRTIO_NET_F_STANDBY feature if the device offers it. +A driver SHOULD accept the VIRTIO_NET_F_CONST_RXBUF_LEN feature if offered. 
+
+If VIRTIO_NET_F_CONST_RXBUF_LEN feature has been negotiated, the
+driver MUST set \field{rx_buf_len}.


I think it's device that set the field?

Makes more sense for the driver, but if you want this set
e.g. before buffers are added, you must say so.


+
+A driver MUST NOT modify \field{rx_buf_len} once it has been set.
+


This seems very unflexible. I can see how e.g. XDP would benefit from
big buffers while skbs benefit from small buffers.

This calls for ability to change this.
Yes, but it requires non trivial cleanups for the old length and place them with new ones. 

Thanks





   \subsubsection{Legacy Interface: Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout / Legacy Interface: Device configuration layout}
   \label{sec:Device Types / Block Device / Feature bits / Device configuration layout / Legacy Interface: Device configuration layout}
   When using the legacy interface, transitional devices and drivers @@
-3241,6 +3257,11 @@ \subsubsection{Setting Up Receive Buffers}\label{sec:Device Types / Network Devi
   If VIRTIO_NET_F_MQ is negotiated, each of receiveq1\ldots receiveqN
   that will be used SHOULD be populated with receive buffers.
+If VIRTIO_NET_F_CONST_RXBUF_LEN feature has been negotiated, the 
+driver MUST initialize all receive virtqueue descriptors \field{len}
+field with the value configured in \field{rx_buf_len} device
+configuration field, and allocate receive buffers accordingly.
+
   \devicenormative{\paragraph}{Setting Up Receive Buffers}{Device
Types / Network Device / Device Operation / Setting Up Receive
Buffers}
The device MUST set \field{num_buffers} to the number of descriptors 
used to @@ -3396,6 +3417,10 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
   checksum (in case of multiple encapsulated protocols, one level
   of checksums is validated).
+If VIRTIO_NET_F_CONST_RXBUF_LEN has been negotiated, the device MAY 
+use \field{rx_buf_len} as a buffer length (instead of reading it from
+virtqueue descriptor \field{len} field).


Is this safe? What if driver submit a small buffer, then device can read more than what is allowed?

Thanks



+
   \drivernormative{\paragraph}{Processing of Incoming
   Packets}{Device Types / Network Device / Device Operation /
   Processing of Incoming Packets}
--


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