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

• From: "Michael S. Tsirkin" <mst@redhat.com>
• To: Jason Wang <jasowang@redhat.com>
• Date: Tue, 3 Mar 2020 09:48:25 -0500

On Thu, Feb 27, 2020 at 06:20:43PM +0800, Jason Wang wrote:
>
> On 2020/2/27 äå6:11, Michael S. Tsirkin wrote:
> > On Thu, Feb 27, 2020 at 06:05:30PM +0800, Jason Wang wrote:
> > > On 2020/2/27 äå1:10, Michael S. Tsirkin wrote:
> > > > 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 äå12:30, 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?
> > > > > >
> > > > > Wouldnât 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.
> > >
> > > I may miss something but I think device won't work correctly if this feature
> > > is not negotiated?
> > >
> > > Thanks
> > That would depend on the device. Device can always fail FEATURES_OK
> > if it doesn't support existing drivers.
> > I really need to address https://github.com/oasis-tcs/virtio-spec/issues/71
> > so people know what to expect ...
>
>
> That's my understanding as well.
>
> E.g for IOMMU_PLATFORM, device can still work if it the feature is not
> negotiated so we use "SHOULD" in the spec.
>
> But for this feature, there's no way except for failing the negotiation, so
> "MUST" seems correct.
>
> Thanks

That would depend on the device. Some devices could work without the
extra data in the kick, just with worse performance.
So I feel this is device's job to enforce this bit,
not driver's.

>
> >
> > > >
> > > > > > 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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