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

[Vitaly Mireyno <vmireyno@marvell.com>]

>-----Original Message-----
>From: Michael S. Tsirkin <mst@redhat.com>
>Sent: Thursday, 5 March, 2020 21:59
>To: Vitaly Mireyno <vmireyno@marvell.com>
>Cc: virtio-comment@lists.oasis-open.org; Jason Wang <jasowang@redhat.com>; Ariel Elior
><aelior@marvell.com>
>Subject: [EXT] Re: [PATCH v3] virtio-net: Add support for the flexible driver notification structure
>
>External Email
>
>----------------------------------------------------------------------
>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?
>

I see your point. Will send a fixed patch.

>
>
>>  \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}
>> --