Subject: [PATCHv3] virtio-spec: virtio network device multiqueue support

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 6 Sep 2012 15:08:29 +0300

Add multiqueue support to virtio network device.
Add a new feature flag VIRTIO_NET_F_MULTIQUEUE for this feature, a new
configuration field max_virtqueue_pairs to detect supported number of
virtqueues as well as a new command VIRTIO_NET_CTRL_STEERING to program
packet steering.

Signed-off-by: Michael S. Tsirkin <[email-masked]>

--

Changes from v2:
Address Jason's comments on v2:
- Changed STEERING_HOST to STEERING_RX_FOLLOWS_TX:
  this is both clearer and easier to support.
  It does not look like we need a separate steering command
  since host can just watch tx packets as they go.
- Moved RX and TX steering sections near each other.
- Add motivation for other changes in v2

Changes from Jason's rfc:
- reserved vq 3: this makes all rx vqs even and tx vqs odd, which
  looks nicer to me.
- documented packet steering, added a generalized steering programming
  command. Current modes are single queue and host driven multiqueue,
  but I envision support for guest driven multiqueue in the future.
- make default vqs unused when in mq mode - this wastes some memory
  but makes it more efficient to switch between modes as
  we can avoid this causing packet reordering.

Rusty, could you please take a look and comment?
If this looks OK to everyone, we can proceed with finalizing the
implementation.  This patch is against
eb9fc84d0d3c46438aaab190e2401a9e5409a052 in virtio-spec git tree.

diff --git a/virtio-spec.lyx b/virtio-spec.lyx
index 7a073f4..a713807 100644
--- a/virtio-spec.lyx
+++ b/virtio-spec.lyx
@@ -58,6 +58,7 @@
 \html_be_strict false
 \author -608949062 "Rusty Russell,,," 
 \author 1531152142 "Paolo Bonzini,,," 
+\author 1986246365 "Michael S. Tsirkin" 
 \end_header
 
 \begin_body
@@ -3896,6 +3897,37 @@ Only if VIRTIO_NET_F_CTRL_VQ set
 \end_inset

+\change_inserted 1986246365 1346663522
+ 3: reserved
+\end_layout
+
+\begin_layout Description
+
+\change_inserted 1986246365 1346663550
+4: receiveq1.
+ 5: transmitq1.
+ 6: receiveq2.
+ 7.
+ transmitq2.
+ ...
+ 2N+2:receivqN, 2N+3:transmitqN
+\begin_inset Foot
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346663558
+Only if VIRTIO_NET_F_CTRL_VQ set.
+ N is indicated by max_virtqueue_pairs field.
+\change_unchanged
+
+\end_layout
+
+\end_inset
+
+
+\change_unchanged
+
 \end_layout
 
 \begin_layout Description
@@ -4056,6 +4088,17 @@ VIRTIO_NET_F_CTRL_VLAN
 
 \begin_layout Description
 VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous packets.
+\change_inserted 1986246365 1346617842
+
+\end_layout
+
+\begin_layout Description
+
+\change_inserted 1986246365 1346618103
+VIRTIO_NET_F_MULTIQUEUE(22) Device has multiple receive and transmission
+ queues.
+\change_unchanged
+
 \end_layout
 
 \end_deeper
@@ -4068,11 +4111,45 @@ configuration
 \begin_inset space ~
 \end_inset
 
-layout Two configuration fields are currently defined.
+layout 
+\change_deleted 1986246365 1346671560
+Two
+\change_inserted 1986246365 1346671647
+Six
+\change_unchanged
+ configuration fields are currently defined.
  The mac address field always exists (though is only valid if VIRTIO_NET_F_MAC
  is set), and the status field only exists if VIRTIO_NET_F_STATUS is set.
  Two read-only bits are currently defined for the status field: VIRTIO_NET_S_LIN
 K_UP and VIRTIO_NET_S_ANNOUNCE.
+
+\change_inserted 1986246365 1346930950
+ The following four read-only fields only exists if VIRTIO_NET_F_MULTIQUEUE
+ is set.
+ The max_virtqueue_pairs field specifies the maximum number of each of transmit
+ and receive virtqueues that can be used for multiqueue operation.
+ The following read-only fields: 
+\emph on
+current_steering_rule
+\emph default
+, 
+\emph on
+reserved
+\emph default
+ and 
+\emph on
+current_steering_param
+\emph default
+ store the last successful VIRTIO_NET_CTRL_STEERING
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "sub:Transmit-Packet-Steering"
+
+\end_inset
+
+ command executed by driver, for debugging purposes.
+
+\change_unchanged
  
 \begin_inset listings
 inline false
@@ -4105,6 +4182,40 @@ struct virtio_net_config {
 \begin_layout Plain Layout
 
     u16 status;
+\change_inserted 1986246365 1346671221
+
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346671532
+
+    u16 max_virtqueue_pairs;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346671531
+
+    u8 current_steering_rule;
+\change_unchanged
+
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346671499
+
+    u8 reserved;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346671530
+
+    u16 current_steering_param;
+\change_unchanged
+
 \end_layout
 
 \begin_layout Plain Layout
@@ -4151,6 +4262,18 @@ physical
 \begin_layout Enumerate
 If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify the control
  virtqueue.
+\change_inserted 1986246365 1346618052
+
+\end_layout
+
+\begin_layout Enumerate
+
+\change_inserted 1986246365 1346618175
+If VIRTIO_NET_F_MULTIQUEUE feature bit is negotiated, identify the receive
+ and transmission queues that are going to be used in multiqueue mode.
+ Only queues that are going to be used need to be initialized.
+\change_unchanged
+
 \end_layout
 
 \begin_layout Enumerate
@@ -4168,7 +4291,11 @@ status
 \end_layout
 
 \begin_layout Enumerate
-The receive virtqueue should be filled with receive buffers.
+The receive virtqueue
+\change_inserted 1986246365 1346618180
+(s)
+\change_unchanged
+ should be filled with receive buffers.
  This is described in detail below in 
 \begin_inset Quotes eld
 \end_inset
@@ -4513,6 +4640,8 @@ Note that the header will be two bytes longer for the VIRTIO_NET_F_MRG_RXBUF
 \end_inset

+\change_deleted 1986246365 1346932640
+
 \end_layout
 
 \begin_layout Subsection*
@@ -4988,8 +5117,24 @@ status open
 The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status field when
  it notices the changes of device configuration.
  The command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that driver
- has recevied the notification and device would clear the VIRTIO_NET_S_ANNOUNCE
- bit in the status filed after it received this command.
+ has rece
+\change_inserted 1986246365 1346663932
+i
+\change_unchanged
+v
+\change_deleted 1986246365 1346663934
+i
+\change_unchanged
+ed the notification and device would clear the VIRTIO_NET_S_ANNOUNCE bit
+ in the status fi
+\change_inserted 1986246365 1346663942
+e
+\change_unchanged
+l
+\change_deleted 1986246365 1346663943
+e
+\change_unchanged
+d after it received this command.
 \end_layout
 
 \begin_layout Standard
@@ -5004,10 +5149,298 @@ Sending the gratuitous packets or marking there are pending gratuitous packets
 \begin_layout Enumerate
 Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control vq.
  
+\change_deleted 1986246365 1346662247
+
 \end_layout
 
-\begin_layout Enumerate
+\begin_layout Subsection*
+
+\change_inserted 1986246365 1346932658
+\begin_inset CommandInset label
+LatexCommand label
+name "sub:Transmit-Packet-Steering"
+
+\end_inset
+
+Transmit Packet Steering
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+When VIRTIO_NET_F_MULTIQUEUE feature bit is negotiated, guest can use any
+ of multiple configured transmit queues to transmit a given packet.
+ To avoid packet reordering by device (which generally leads to performance
+ degradation) driver should attempt to utilize the same transmit virtqueue
+ for all packets of a given transmit flow.
+ For bi-directional protocols (in practice, TCP), a given network connection
+ can utilize both transmit and receive queues.
+ For best performance, packets from a single connection should utilize the
+ paired transmit and receive queues from the same virtqueue pair; for example
+ both transmitqN and receiveqN.
+ This rule makes it possible to optimize processing on the device side,
+ but this is not a hard requirement: devices should function correctly even
+ when this rule is not followed.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+Driver selects an active steering rule using VIRTIO_NET_CTRL_STEERING command
+ (this controls both which virtqueue is selected for a given packet for
+ receive and notifies the device which virtqueues are about to be used for
+ transmit).
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+This command accepts a single out argument in the following format:
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+\begin_inset listings
+inline false
+status open
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+#define VIRTIO_NET_CTRL_STEERING       4
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+struct virtio_net_ctrl_steering {
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+	u8 current_steering_rule;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+    u8 reserved;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+	u16 current_steering_param;
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+};
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+#define VIRTIO_NET_CTRL_STEERING_SINGLE       0
+\end_layout
+
+\begin_layout Plain Layout
+
+\change_inserted 1986246365 1346932658
+
+#define VIRTIO_NET_CTRL_STEERING_HOST  1
+\end_layout
+
+\end_inset
+
+
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+The field 
+\emph on
+rule
+\emph default
+ specifies the function used to select transmit virtqueue for a given packet;
+ the field 
+\emph on
+param
+\emph default
+ makes it possible to pass an extra parameter if appropriate.
+ When 
+\emph on
+rule
+\emph default
+ is set to VIRTIO_NET_CTRL_STEERING_SINGLE (this is the default) all packets
+ are steered to the default virtqueue transmitq (1); param is unused; this
+ is the default.
+ With any other rule, When 
+\emph on
+rule
+\emph default
+ is set to VIRTIO_NET_CTRL_STEERING_RX_FOLLOWS_TX packets are steered by
+ driver to the first (
+\emph on
+param
+\emph default
++1) multiqueue virtqueues transmitq1...transmitqN; the default transmitq is
+ unused.
+ Driver must have configured all these (
+\emph on
+param
+\emph default
++1) virtqueues beforehand.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+Supported steering rules can be added and removed in the future.
+ Driver should check that the request to change the steering rule was successful
+ by checking ack values of the command.
+ As selecting a specific steering ais n optimization feature, drivers should
+ avoid hard failure and fall back on using a supported steering rule if
+ this command fails.
+ The default steering rule is VIRTIO_NET_CTRL_STEERING_SINGLE.
+ It will not be removed.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+When the steering rule is modified, some packets can still be outstanding
+ in one or more of the transmit virtqueues.
+ Since drivers might choose to modify the current steering rule at a high
+ rate (e.g.
+ adaptively in response to changes in the workload) to avoid reordering
+ packets, device is recommended to complete processing of the transmit queue(s)
+ utilized by the original steering before processing any packets delivered
+ by the modified steering rule.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932658
+For debugging, the current steering rule can also be read from the configuration
+ space.
+\end_layout
+
+\begin_layout Subsection*
+
+\change_inserted 1986246365 1346670357
+\begin_inset CommandInset label
+LatexCommand label
+name "sub:Receive-Packet-Steering"
+
+\end_inset
+
+Receive Packet Steering
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346671046
+When VIRTIO_NET_F_MULTIQUEUE feature bit is negotiated, device can use any
+ of multiple configured receive queues to pass a given packet to driver.
+ Driver controls which virtqueue is selected in practice by configuring
+ packet steering rule using VIRTIO_NET_CTRL_STEERING command, as described
+ above
+\begin_inset CommandInset ref
+LatexCommand ref
+reference "sub:Transmit-Packet-Steering"
+
+\end_inset
+
+.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346931532
+The field 
+\emph on
+rule
+\emph default
+ specifies the function used to select receive virtqueue for a given packet;
+ the field 
+\emph on
+param
+\emph default
+ makes it possible to pass an extra parameter if appropriate.
+ When 
+\emph on
+rule
+\emph default
+ is set to VIRTIO_NET_CTRL_STEERING_SINGLE all packets are steered to the
+ default virtqueue receveq (0); param is unused; this is the default.
+ When 
+\emph on
+rule
+\emph default
+ is set to VIRTIO_NET_CTRL_STEERING_RX_FOLLOWS_TX packets are steered by
+ host to the first (
+\emph on
+param
+\emph default
++1) multiqueue virtqueues receiveq1...receiveqN; the default receiveq is unused.
+ Driver must have configured all these (
+\emph on
+param
+\emph default
++1) virtqueues beforehand.
+ For best performance for bi-directional flows (such as TCP) device should
+ detect the flow to virtqueue pair mapping on transmit and select the receive
+ virtqueue from the same virtqueue pair.
+ For uni-directional flows, or when this mapping information is missing,
+ a device-specific steering function is used.
+\change_unchanged
+
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346669564
+Supported steering rules can be added and removed in the future.
+ Driver should probe for supported rules by checking ack values of the command.
+\end_layout
+
+\begin_layout Standard
+
+\change_inserted 1986246365 1346932135
+When the steering rule is modified, some packets can still be outstanding
+ in one or more of the virtqueues.
+ Device is not required to wait for these packets to be consumed before
+ delivering packets using the new streering rule.
+ Drivers modifying the steering rule at a high rate (e.g.
+ adaptively in response to changes in the workload) are recommended to complete
+ processing of the receive queue(s) utilized by the original steering before
+ processing any packets delivered by the modified steering rule.
+\end_layout
+
+\begin_layout Standard
+
+\change_deleted 1986246365 1346664095
 .
+
+\change_unchanged
  
 \end_layout