[virtio-comment] [PATCH REQUIREMENTS v2 2/7] net-features: Add low latency transmit queue requirements

[Parav Pandit <parav@nvidia.com>]

Add requirements for the low latency transmit queue.

Signed-off-by: Parav Pandit <parav@nvidia.com>
---
chagelog:
v0->v1:
- added design goals for which requirements are added
---
 net-workstream/features-1.4.md | 81 ++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

diff --git a/net-workstream/features-1.4.md b/net-workstream/features-1.4.md
index 4c3797b..0c3202c 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
@@ -33,3 +34,83 @@ together is desired while updating the virtio net interface.
 ### 3.1.2 Per transmit queue counters
 1. le64 tx_gso_pkts: Packets send as transmit GSO sequence
 2. le64 tx_pkts: Total packets send by the device
+
+## 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 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 vitio_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 place all transmit completion together with it per packet stream
+   transmit timestamp using single PCIe transcation.
-- 
2.26.2


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