Re: [virtio-comment] [PATCH v3] virtio-net: Add support for the flexible driver notification structure

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

On Wed, Feb 26, 2020 at 02:54:36PM +0000, Vitaly Mireyno wrote:
> 
> >-----Original Message-----
> >From: Jason Wang <jasowang@redhat.com>
> >Sent: Wednesday, 26 February, 2020 5:13
> >To: Vitaly Mireyno <vmireyno@marvell.com>; virtio-comment@lists.oasis-open.org
> >Cc: Michael S. Tsirkin <mst@redhat.com>; Ariel Elior <aelior@marvell.com>
> >Subject: [EXT] Re: [virtio-comment] [PATCH v3] virtio-net: Add support for the flexible driver
> >notification structure
> >
> >----------------------------------------------------------------------
> >
> >On 2020/2/26 上午12:30, Vitaly Mireyno wrote:
> >> Currently, the driver notification (available buffer notification) has a fixed structure.
> >> If VIRTIO_F_NOTIFICATION_DATA has been negotiated, it includes: vqn, next_off and next_wrap.
> >> If notify_off_multiplier > 0, the VQ number can be derived by the device from the Queue Notify
> >address, so vqn may be redundant.
> >>
> >> Some devices benefit from receiving an additional data with driver notifications. This data can
> >optionally replace the vqn field in the driver notification structure.
> >> In its simplest form, it would be sufficient for this data to be a per-device constant value.
> >>
> >> Changes from v2 - Defined a new feature flag instead of a PCI-specific flag.
> >>
> >> Signed-off-by: Vitaly Mireyno <vmireyno@marvell.com>
> >> ---
> >>   content.tex | 34 ++++++++++++++++++++++++++++++++++
> >>   1 file changed, 34 insertions(+)
> >>
> >> diff --git a/content.tex b/content.tex index b91a132..5223d5c 100644
> >> --- a/content.tex
> >> +++ b/content.tex
> >> @@ -965,6 +965,8 @@ \subsubsection{Notification structure layout}\label{sec:Virtio Transport
> >Options
> >>   struct virtio_pci_notify_cap {
> >>           struct virtio_pci_cap cap;
> >>           le32 notify_off_multiplier; /* Multiplier for
> >> queue_notify_off. */
> >> +        le16 notify_data; /* Data to be placed in the vqn field */
> >> +        le16 padding; /* Pad to a dword */
> >>   };
> >>   \end{lstlisting}
> >>
> >> @@ -984,6 +986,21 @@ \subsubsection{Notification structure layout}\label{sec:Virtio Transport
> >Options
> >>   the same Queue Notify address for all queues.
> >>   \end{note}
> >>
> >> +\field{notify_data} is the data that the driver will set in the
> >> +\field{vqn} field in the available buffer notification, if
> >> +VIRTIO_NET_F_NOTIF_EXTRA_DATA has been negotiated.
> >> +
> >> +\begin{note}
> >> +If \field{notify_off_multiplier} > 0, the virtqueue number can
> >> +potentially be derived by the device from the Queue Notify address,
> >> +so \field{vqn} may be redundant. Some devices benefit from receiving
> >> +the additional data with driver notifications. An example could be a
> >> +hardware device implementing multiple protocols (with virtio being
> >> +one of them), so extra notification data could serve as a notification type indication or a protocol
> >indication.
> >> +Another example could be using shared hardware memory space for
> >> +driver notifications for multiple virtio devices in a trusted environment.
> >> +\end{note}
> >> +
> >>   \devicenormative{\paragraph}{Notification capability}{Virtio Transport Options / Virtio Over PCI Bus
> >/ PCI Device Layout / Notification capability}
> >>   The device MUST present at least one notification capability.
> >>
> >> @@ -1020,6 +1037,10 @@ \subsubsection{Notification structure layout}\label{sec:Virtio Transport
> >Options
> >>   cap.length >= queue_notify_off * notify_off_multiplier + 4
> >>   \end{lstlisting}
> >>
> >> +If VIRTIO_NET_F_NOTIF_EXTRA_DATA has been negotiated, the device MUST
> >> +set \field{notify_data} to a valid value, and SHOULD set
> >> +\field{notify_off_multiplier} > 0.
> >> +
> >>   \subsubsection{ISR status capability}\label{sec:Virtio Transport
> >> Options / Virtio Over PCI Bus / PCI Device Layout / ISR status
> >> capability}
> >>
> >>   The VIRTIO_PCI_CAP_ISR_CFG capability @@ -1519,6 +1540,14 @@
> >> \subsubsection{Available Buffer Notifications}\label{sec:Virtio Transport Option
> >>   See \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification
> >capability}
> >>   for how to calculate the Queue Notify address.
> >>
> >> +\drivernormative{\paragraph}{Available Buffer Notifications}{Virtio
> >> +Transport Options / Virtio Over PCI Bus / PCI-specific Initialization
> >> +And Device Operation / Available Buffer Notifications} The driver SHOULD accept the
> >VIRTIO_NET_F_NOTIF_EXTRA_DATA feature if it has been offered.
> >
> >
> >I think we need use "MUST" here?
> >
> 
> Wouldn’t it break the existing drivers then?

I agree, we can't declare existing drivers non-conformant by fiat.
Certainly not for a boutique feature like this.


> 
> >Other looks good to me.
> >
> >Thanks
> >
> >
> >> +
> >> +If VIRTIO_NET_F_NOTIF_EXTRA_DATA has been negotiated, the driver MUST
> >> +set the \field{vqn} field of the available buffer notification
> >> +structure to the \field{notify_data} value.
> >> +
> >>   \subsubsection{Used Buffer Notifications}\label{sec:Virtio Transport
> >> Options / Virtio Over PCI Bus / PCI-specific Initialization And Device
> >> Operation / Used Buffer Notifications}
> >>
> >>   If a used buffer notification is necessary for a virtqueue, the device would typically act as follows:
> >> @@ -2895,6 +2924,10 @@ \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_NOTIF_EXTRA_DATA(57)] Driver provides an extra data with
> >> +    available buffer notifications, to aid in notification processing by the
> >> +    device.
> >> +
> >>   \item[VIRTIO_NET_F_GUEST_HDRLEN(59)] Driver can provide the exact \field{hdr_len}
> >>       value. Device benefits from knowing the exact header length.
> >>
> >> @@ -2934,6 +2967,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types /
> >Network Device
> >>   \item[VIRTIO_NET_F_CTRL_MAC_ADDR] Requires VIRTIO_NET_F_CTRL_VQ.
> >>   \item[VIRTIO_NET_F_RSC_EXT] Requires VIRTIO_NET_F_HOST_TSO4 or
> >VIRTIO_NET_F_HOST_TSO6.
> >>   \item[VIRTIO_NET_F_RSS] Requires VIRTIO_NET_F_CTRL_VQ.
> >> +\item[VIRTIO_NET_F_NOTIF_EXTRA_DATA] Requires VIRTIO_F_NOTIFICATION_DATA.
> >>   \end{description}
> >>
> >>   \subsubsection{Legacy Interface: Feature bits}\label{sec:Device
> >> Types / Network Device / Feature bits / Legacy Interface: Feature
> >> bits}
> >> --
> >>
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