On Tuesday, 2023-08-15 at 10:45:55 +03, Parav Pandit wrote:
> Add requirements for the low latency transmit queue.
>
> Signed-off-by: Parav Pandit <>
Acked-by: David Edmondson <>
> ---
> 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.
The presence of "L4 data" seems like a distraction - just give an
indication of size ranges being considered.
> +
> +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.
--
And you're standing here beside me, I love the passing of time.
