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

[David Edmondson <david.edmondson@oracle.com>]

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 <parav@nvidia.com>

Acked-by: David Edmondson <david.edmondson@oracle.com>

> ---
> 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.

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