On Monday, 2023-07-24 at 06:34:16 +03, Parav Pandit wrote:
> Add requirements for the low latency transmit queue. >
> Signed-off-by: Parav Pandit <> > ---
> chagelog:
> v1->v2: > - added generic requirement to inline the request content
>
along with the descriptor for non virtio-net devices > - added requirement to inline the request 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 4c3797b..eb95592 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 +34,90 @@ 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 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 vitio_packed_q_desc { "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 place all transmit completion together with it per packet stream > +
transmit timestamp using single PCIe transcation. The meaning of this is unclear to me. Is it: The ability to place all transmit completions with a per-packet stream transmit timestamp using a single PCIe transaction. ? > + > +8. A generic feature of the virtqueue, to contain such header data inline for virtio > +
devices other than virtio-net. Given that this feature is used by this patch (for TX), the following patch (for RX) and flow filter manipulation, perhaps pull it out as a distinct requirement. > + > +9. A flow filter virtqueue also similarly need the ability to inline the short flow > +
command header.
--
So tap at my window, maybe I might let you in.
