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Subject: Re: [PATCH v3] virtio-net: Add support for the flexible driver notification structure
On Tue, Feb 25, 2020 at 04:30:56PM +0000, 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, I'm not sure what would a valid value mean. Does this requirement add any value (pun intended :) )? > and SHOULD set > +\field{notify_off_multiplier} > 0. So I have a concern here. There could be multiple capabilities. What if some have notify_off_multiplier and some aren't? This is SHOULD so it seems to be legal. Then below ... > + > \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. > + > +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. > + So if notify_off_multiplier for a given capability is actually 0, when using this specific capability, I think we need to keep vqn there as previously. How about listing this behaviour out explicitly? > \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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