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

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

On Mon, Dec 30, 2019 at 01:59:17PM +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.
> This is a follow-up to the "[PATCH] virtio-net: Add equal-sized receive buffers feature" discussion: https://lists.oasis-open.org/archives/virtio-comment/201912/msg00007.html
> 
> 
> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
> ---
>  content.tex | 41 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/content.tex b/content.tex
> index d68cfaf..0a4cfba 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -2815,6 +2815,13 @@ \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_RATIO(57)] Device requests to limit the
> +    maximum/minimum receive buffers length ratio.
> +
> +\item[VIRTIO_NET_F_RXBUF_MIN_LEN(58)] Device requests to know the minimum
> +    receive buffers length. 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.
>  

So I am wondering, when we are switching from regular skbs to XDP,
all buffers become 4K. When we switch back, we get variability again.
All the while some old buffers can be outstanding.

How will we handle this?


> @@ -2861,8 +2868,8 @@ \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 +2889,22 @@ \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 device-read-only field \field{min_rx_buf_len} only exists if
> +VIRTIO_NET_F_RXBUF_MIN_LEN 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_RATIO is set. This field specifies the maximum/minimum receive
> +buffers length 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 +2933,15 @@ \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_MIN_LEN feature if offered.
> +
> +If VIRTIO_NET_F_RXBUF_MIN_LEN feature has been negotiated, the driver MUST
> +set \field{min_rx_buf_len}.
> +
> +A driver MUST NOT modify \field{min_rx_buf_len} once it has been set.
> +
> +A driver SHOULD accept the VIRTIO_NET_F_RXBUF_LEN_RATIO feature if offered.
> +
>  \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.
> @@ -3281,6 +3307,17 @@ \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_MIN_LEN 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 VIRTIO_NET_F_RXBUF_LEN_RATIO feature has been negotiated, the driver
> +MUST initialize all receive virtqueue descriptors \field{len} field with
> +the value less than or equal to (\field{min_rx_buf_len} * \field{rx_buf_len_ratio}),
> +and allocate receive buffers accordingly.
> +
>  \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
> --
> 
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