[PATCH v5] virtio-net: clarify coalescing parameters settings

Heng Qi
2024-05-28T04:47:13Z

The device can set any initial coalescing parameters (0 or non-zero)
for the receive/send queue before the setting command is executed,
not just 0, enhancing device performance even without DIM enabled.

So we need to clarify descriptions that don't fit the behavior.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/194
Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
---
v4->v5:
- Add RB tags from Parav and Jason. Thanks!

v3->v4:
- Doesn't force the device to remember more stuff. @Parav

v2->v3:
- Clarify description to be more generic. @Parav

v1->v2:
- Update description. @Jason

device-types/net/description.tex | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index 61cce1f..00c1b36 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -1803,6 +1803,14 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
for an enabled transmit/receive virtqueue whose index is \field{vq_index}.
\end{enumerate}

+If the VIRTIO_NET_F_NOTF_COAL or VIRTIO_NET_F_VQ_NOTF_COAL feature is negotiated,
+the device may apply any coalescing parameters to each transmit/receive virtqueue
+before the driver successfully performs one of the VIRTIO_NET_CTRL_NOTF_COAL set commands.
+
+The driver can query the coalescing parameters of any enabled transmit/receive
+virtqueue using the VIRTIO_NET_CTRL_NOTF_COAL_VQ_GET command, before or after any
+VIRTIO_NET_CTRL_NOTF_COAL set command is done.
+
The device may generate notifications more or less frequently than specified by set commands of the VIRTIO_NET_CTRL_NOTF_COAL class.

If coalescing parameters are being set, the device applies the last coalescing parameters set for a
@@ -1885,17 +1893,17 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
VIRTIO_NET_ERR if the designated virtqueue is not an enabled transmit or receive virtqueue.

Upon disabling and re-enabling a transmit virtqueue, the device MUST set the coalescing parameters of the virtqueue
-to those configured through the VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command, or, if the driver did not set any TX coalescing parameters, to 0.
+to those configured through the VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command, or, if the driver did not set any TX coalescing parameters, the device MAY initialize them to any values.

Upon disabling and re-enabling a receive virtqueue, the device MUST set the coalescing parameters of the virtqueue
-to those configured through the VIRTIO_NET_CTRL_NOTF_COAL_RX_SET command, or, if the driver did not set any RX coalescing parameters, to 0.
+to those configured through the VIRTIO_NET_CTRL_NOTF_COAL_RX_SET command, or, if the driver did not set any RX coalescing parameters, the device MAY initialize them to any values.

The behavior of the device in response to set commands of the VIRTIO_NET_CTRL_NOTF_COAL class is best-effort:
the device MAY generate notifications more or less frequently than specified.

A device SHOULD NOT send used buffer notifications to the driver if the notifications are suppressed, even if the notification conditions are met.

-Upon reset, a device MUST initialize all coalescing parameters to 0.
+Upon reset, a device MAY set any coalescing parameters for all transmit and receive virtqueues.

\paragraph{Device Statistics}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Device Statistics}

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2.32.0.3.g01195cf9f