Re: [virtio] [PATCH REQUIREMENTS v2 4/7] net-features: Add notification coalescing requirements

[Stefan Hajnoczi <stefanha@redhat.com>]

On Mon, Jul 03, 2023 at 02:44:07AM +0300, Parav Pandit wrote:
> Add virtio net device notification coalescing improvements requirements.
> 
> Signed-off-by: Parav Pandit <parav@nvidia.com>
> ---
> changelog:
> v0->v1:
> - updated the description
> ---
>  net-workstream/features-1.4.md | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/net-workstream/features-1.4.md b/net-workstream/features-1.4.md
> index 3e8b5a4..a34556c 100644
> --- a/net-workstream/features-1.4.md
> +++ b/net-workstream/features-1.4.md
> @@ -8,6 +8,7 @@ together is desired while updating the virtio net interface.
>  # 2. Summary
>  1. Device counters visible to the driver
>  2. Low latency tx and rx virtqueues for PCI transport
> +3. Virtqueue notification coalescing re-arming support
>  
>  # 3. Requirements
>  ## 3.1 Device counters
> @@ -157,3 +158,14 @@ struct vnet_rx_completion {
>     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.
> +
> +## 3.3 Virtqueue notification coalescing re-enable support

It's called "re-arming" above but "re-enable" here. Please choose one
term and use it consistently.

> +0. Design goal:
> +   a. Avoid constant notification from the device even in conditions when
> +      the driver may not have acted on the previous pending notification.
> +1. When Tx and Rx virtqueue notification coalescing is enabled, and when such
> +   notification is reported by the device, device should be able to disable

"notification" -> "a notification"

", device" -> ", the device"

> +   further notifications until the driver finish reacting to the already

s/finish/finishes/

> +   generated notification. 
> +2. When the driver enables the notification coalescing reporting, the device

"enables the notification coalescing reporting" ->
"enables notification coalescing reporting"

> +   to notify again if notification coalescing conditions are met.

I can't parse this sentence. Maybe "the device _has_ to notify again
..."?

I find this text hard to understand. Is this a mechanism where the
device does not send further notifications on a virtqueue until the
driver has re-armed them?

How does this relate to EVENT_IDX, which can be used to achieve a
similar effect? I guess the downside to EVENT_IDX is that the device
must DMA repeatedly in order to detect changes from driver, whereas this
new re-arming mechanism involves a hardware register write?

Can this new mechanism be generic for any kind of virtqueue, not just
virtio-net rx/tx?

> -- 
> 2.26.2
> 
> 
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