Add requirements for the low latency transmit queue.
Signed-off-by: Parav Pandit <>
---
chagelog:
v3->v4:
- Addressed comments from David
- rewrote timestamp and completions pcie transcation requirement
v1->v2:
- 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
v0->v1:
- 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.
--
2.26.2
