Re: [PATCH v2] virtio-net: Add an optional device control over the receive buffers length

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 04, 2020 at 04:13:36PM +0000, Vitaly Mireyno wrote:
> This patch gives devices some level of control over the receive buffers length.
> The driver declares the minimum receive buffer length, and the device requests max/min buffer length ratio.
> 
> v2 incorporates v1 comments.
> 
> Changes from v1:
>  * min_rx_buf_len must be set before FEATURES_OK
>  * Use a single feature bit VIRTIO_NET_F_RXBUF_LEN_CTRL for both, the min size and the ratio
>  * min_rx_buf_len is defined as write-only for driver
> 
> 
> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
> ---
>  content.tex | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index d68cfaf..ab0d193 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -2815,6 +2815,10 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
>  \item[VIRTIO_NET_F_CTRL_MAC_ADDR(23)] Set MAC address through control
>      channel.
>  
> +\item[VIRTIO_NET_F_RXBUF_LEN_CTRL(58)] Device requests to know the minimum
> +    receive buffers length, and requests to limit the maximum/minimum receive
> +    buffers length ratio. Driver declares the minimum receive buffers length.
> +
>  \item[VIRTIO_NET_F_GUEST_HDRLEN(59)] Driver can provide the exact \field{hdr_len}
>      value. Device benefits from knowing the exact header length.
>  
> @@ -2861,8 +2865,8 @@ \subsubsection{Legacy Interface: Feature bits}\label{sec:Device Types / Network
>  \subsection{Device configuration layout}\label{sec:Device Types / Network Device / Device configuration layout}
>  \label{sec:Device Types / Block Device / Feature bits / Device configuration layout}
>  
> -Three driver-read-only configuration fields are currently defined. The \field{mac} address field
> -always exists (though is only valid if VIRTIO_NET_F_MAC is set), and
> +The driver-read-only \field{mac} address field always exists
> +(though is only valid if VIRTIO_NET_F_MAC is set), and
>  \field{status} only exists if VIRTIO_NET_F_STATUS is set. Two
>  read-only bits (for the driver) are currently defined for the status field:
>  VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE.
> @@ -2882,12 +2886,22 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
>  VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the driver to
>  use.
>  
> +The write-only for driver field \field{min_rx_buf_len} only exists if
> +VIRTIO_NET_F_RXBUF_LEN_CTRL is set. This field specifies the minimum receive
> +buffers length.
> +
> +The driver-read-only field \field{rx_buf_len_ratio} only exists if
> +VIRTIO_NET_F_RXBUF_LEN_CTRL is set. This field specifies the maximum/minimum receive
> +buffers length ratio. The value '0' indicates unrestricted ratio.
> +
>  \begin{lstlisting}
>  struct virtio_net_config {
>          u8 mac[6];
>          le16 status;
>          le16 max_virtqueue_pairs;
>          le16 mtu;
> +        le32 min_rx_buf_len;
> +        le16 rx_buf_len_ratio;
>  };
>  \end{lstlisting}
>  
> @@ -2916,6 +2930,13 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
>  If the driver negotiates the VIRTIO_NET_F_STANDBY feature, the device MAY act
>  as a standby device for a primary device with the same MAC address.
>  
> +A driver SHOULD accept the VIRTIO_NET_F_RXBUF_LEN_CTRL feature if offered.
> +
> +If VIRTIO_NET_F_RXBUF_LEN_CTRL feature has been negotiated, the driver MUST
> +set \field{min_rx_buf_len} before setting the FEATURES_OK status bit.
> +
> +A driver MUST NOT modify \field{min_rx_buf_len} once it has been set.
> +
>  \drivernormative{\subsubsection}{Device configuration layout}{Device Types / Network Device / Device configuration layout}
>  
>  A driver SHOULD negotiate VIRTIO_NET_F_MAC if the device offers it.

I still think this is too restrictive. Switching between e.g.
XDP/non-XDP workloads demands ability to change this
without device reset. I am thinking about sending
the avail index to device with a command to change
the size, this way device can figure out where are the old
and where are the new buffers.
Doesn't look too hard on the hardware, does it?

> @@ -3281,6 +3302,13 @@ \subsubsection{Setting Up Receive Buffers}\label{sec:Device Types / Network Devi
>  If VIRTIO_NET_F_MQ is negotiated, each of receiveq1\ldots receiveqN
>  that will be used SHOULD be populated with receive buffers.
>  
> +If VIRTIO_NET_F_RXBUF_LEN_CTRL feature has been negotiated, the driver
> +MUST initialize all receive virtqueue descriptors \field{len} field with
> +the value greater than or equal to the value configured in the
> +\field{min_rx_buf_len} device configuration field, and allocate receive
> +buffers accordingly. If \field{rx_buf_len_ratio} > 0, then the \field{len} value
> +must also be less than or equal to (\field{min_rx_buf_len} * \field{rx_buf_len_ratio}).
> +
>  \devicenormative{\paragraph}{Setting Up Receive Buffers}{Device Types / Network Device / Device Operation / Setting Up Receive Buffers}
>  
>  The device MUST set \field{num_buffers} to the number of descriptors used to
> --