[virtio-comment] [PATCH requirements v5 3/7] net-features: Add low latency receive queue requirements

[Parav Pandit <parav@nvidia.com>]

Add requirements for the low latency receive queue.

Signed-off-by: Parav Pandit <parav@nvidia.com>
---
changelog:
v0->v1:
- clarified the requirements further
- added line for the gro case
- added design goals as the motivation for the requirements
---
 net-workstream/features-1.4.md | 45 +++++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/net-workstream/features-1.4.md b/net-workstream/features-1.4.md
index 1167ce2..bc9e971 100644
--- a/net-workstream/features-1.4.md
+++ b/net-workstream/features-1.4.md
@@ -7,7 +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
+2. Low latency tx and rx virtqueues for PCI transport
 
 # 3. Requirements
 ## 3.1 Device counters
@@ -127,3 +127,46 @@ struct vnet_data_desc desc[2];
 
 9. A flow filter virtqueue also similarly need the ability to inline the short flow
    command header.
+
+### 3.2.2 Low latency rx virtqueue
+0. Design goal:
+   a. Keep packet metadata and buffer data together which is consumed by driver
+      layer and make it available in a single cache line of cpu
+   b. Instead of having per packet descriptors which is complex to scale for
+      the device, supply the page directly to the device to consume it based
+      on packet size
+1. The device should be able to write a packet receive completion that consists
+   of struct virtio_net_hdr (or similar) and a buffer id using a single DMA write
+   PCIe TLP.
+2. The device should be able to perform DMA writes of multiple packets
+   completions in a single DMA transaction up to the PCIe maximum write limit
+   in a transaction.
+3. The device should be able to zero pad packet write completion to align it to
+   64B or CPU cache line size whenever possible.
+4. An example of the above DMA completion structure:
+
+```
+/* Constant size receive packet completion */
+struct vnet_rx_completion {
+   u16 flags;
+   u16 id; /* buffer id */
+   u8 gso_type;
+   u8 reserved[3];
+   le16 gso_hdr_len;
+   le16 gso_size;
+   le16 csum_start;
+   le16 csum_offset;
+   u16 reserved2;
+   u64 timestamp; /* explained later */
+   u8 padding[];
+};
+```
+5. The driver should be able to post constant-size buffer pages on a receive
+   queue which can be consumed by the device for an incoming packet of any size
+   from 64B to 9K bytes.
+6. The device should be able to know the constant buffer size at receive
+   virtqueue level instead of per buffer level.
+7. The device should be able to indicate when a full page buffer is consumed,
+   which can be recycled by the driver when the packets from the completed
+   page is fully consumed.
+8. The device should be able to consume multiple pages for a receive GSO stream.
-- 
2.26.2


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