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