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Subject: [PATCH requirements v4 2/7] net-features: Add low latency transmit queue requirements

Add requirements for the low latency transmit queue.

Signed-off-by: Parav Pandit <parav@nvidia.com>
- Addressed comments from David
- rewrote timestamp and completions pcie transcation requirement
- added generic requirement to inline the request content
  along with the descriptor for non virtio-net devices
- added requirement to inline the header content along
  with the descriptor for virtio flow filter queue as two
  features are similar
- added design goals for which requirements are added
 net-workstream/features-1.4.md | 88 ++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)

diff --git a/net-workstream/features-1.4.md b/net-workstream/features-1.4.md
index c2b1cc8..40fa07f 100644
--- a/net-workstream/features-1.4.md
+++ b/net-workstream/features-1.4.md
@@ -7,6 +7,7 @@ together is desired while updating the virtio net interface.
 # 2. Summary
 1. Device counters visible to the driver
+2. Low latency tx virtqueue for PCI transport
 # 3. Requirements
 ## 3.1 Device counters
@@ -41,3 +42,90 @@ together is desired while updating the virtio net interface.
 More counters discussed in [1].
 [1] https://lists.oasis-open.org/archives/virtio-comment/202308/msg00176.html
+## 3.2 Low PCI latency virtqueues
+### 3.2.1 Low PCI latency tx virtqueue
+0. Design goal
+   a. Reduce PCI access latency in packet transmit flow
+   b. Avoid O(N) descriptor parser to detect a packet stream to simplify device
+      logic
+   c. Reduce number of PCI transmit completion transactions and have unified
+      completion flow with/without transmit timestamping
+   d. Avoid partial cache line writes on transmit completions
+1. Packet transmit descriptor should contain data descriptors count without any
+   indirection and without any O(N) search to find the end of a packet stream.
+   For example, a packet transmit descriptor (called vnet_tx_hdr_desc
+   subsequently) to contain a field num_next_desc for the packet stream
+   indicating that a packet is located in N data descriptors.
+2. Packet transmit descriptor should contain segmentation offload-related fields
+   without any indirection. For example, packet transmit descriptor to contain
+   gso_type, gso_size/mss, header length, csum placement byte offset, and
+   csum start.
+3. Packet transmit descriptor should be able to place a small size packet that
+   does not have any L4 data after the vnet_tx_hdr_desc in the virtqueue memory.
+   For example a TCP ack only packet can fit in a descriptor memory which
+   otherwise consume more than 25% of metadata to describe the packet.
+4. Packet transmit descriptor should be able to place a full GSO header (L2 to
+   L4) after header descriptor and before data descriptors. For example, the
+   GSO header is placed after struct vnet_tx_hdr_desc in the virtqueue memory.
+   When such a GSO header is positioned adjacent to the packet transmit
+   descriptor, and when the GSO header is not aligned to 16B, the following
+   data descriptor to start on the 8B aligned boundary.
+5. An example of the above requirements at high level is:
+struct virtio_packed_q_desc {
+   /* current desc for reference */
+   u64 address;
+   u32 len;
+   u16 id;
+   u16 flags;
+/* Constant size header descriptor for tx packets */
+struct vnet_tx_hdr_desc {
+   u16 flags; /* indicate how to parse next fields */
+   u16 id; /* desc id to come back in completion */
+   u8 num_next_desc; /* indicates the number of the next 16B data desc for this
+		      * buffer.
+		      */
+   u8 gso_type;
+   le16 gso_hdr_len;
+   le16 gso_size;
+   le16 csum_start;
+   le16 csum_offset;
+   u8 inline_pkt_len; /* indicates the length of the inline packet after this
+		       * desc
+		       */
+   u8 reserved;
+   u8 padding[];
+/* Example of a short packet or GSO header placed in the desc section of the vq
+ */
+struct vnet_tx_small_pkt_desc {
+   u8 raw_pkt[128];
+/* Example of header followed by data descriptor */
+struct vnet_tx_hdr_desc hdr_desc;
+struct vnet_data_desc desc[2];
+6. Ability to zero pad the transmit completion when the transmit completion is
+   shorter than the CPU cache line size.
+7. Ability to write per packet timestamp and also write multiple
+   transmit completions using single PCIe transcation.
+8. A generic feature of the virtqueue, to contain such header data inline for virtio
+   devices other than virtio-net.
+9. A flow filter virtqueue also similarly need the ability to inline the short flow
+   command header.

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