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Subject: Re: [virtio-comment] [PATCH V2] virtio-net: introduce admin control virtqueue

On Mon, 25 Jan 2021 13:52:02 +0800
Jason Wang <jasowang@redhat.com> wrote:

> When implementing virtual devices like SR-IOV or sub-function. We're
> suffering from several issues:
> - There's no management interface for management device to
>   configure features, attributes for a virtual device
> - Per virtual device control virtqueue could be very expensive as the
>   number of virtual devices could be very large
> - Virtualize per virtual device's control virtqueue could be very
>   challenge as we need the support of DMA isolation at queue level
> So this patch introduces the feature of VIRTIO_NET_CTRL_ADMIN_VQ. This
> allows the device to implement a single admin control virtqueue to
> manage the features and attributes for a specific virtual device.
> The idea is simple, a new virtual device id is introduced on top of
> the existing virtio_net_ctrl structure. This id is transport or device
> specific to uniquely identify a management or virtual device.
> With this, we get a way of using management device (PF) to configure
> per virtual device features and attributes. And since the admin
> control virtqueue belongs to management device (PF), the DMA is
> naturally isolated at device level instead of the queue level for per
> virtual device control vq.
> When the admin cvq is offered by management device and normal cvq is
> offered by virtual device. A new command class is introduced decide
> whether or not to accept commands from normal cvq for a virtual
> device.

First, sorry for being late. Second, I'm still struggling with putting
my mental model together. I will ask my questions at the correspondig

> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
> Changes from V1:
> - use 'virtual device' instead of 'function'
> - introuce trust command
> - clairfy that the admin cvq could be used to configure management
>   device itself
> ---
>  content.tex | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 56 insertions(+), 3 deletions(-)
> diff --git a/content.tex b/content.tex
> index 620c0e2..989b4f6 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -2940,6 +2940,9 @@ \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_ADMIN_CTRL_VQ(56)] Admin control channel is
> +    available.
> +
>  \item[VIRTIO_NET_F_HASH_REPORT(57)] Device can report per-packet hash
>      value and a type of calculated hash.
> @@ -3840,11 +3843,12 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
>  \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue}
>  The driver uses the control virtqueue (if VIRTIO_NET_F_CTRL_VQ is
> -negotiated) to send commands to manipulate various features of
> -the device which would not easily map into the configuration
> -space.
> +negotiated but VIRTIO_NET_F_ADMIN_CTRL_VQ is not negotiated) to send
> +commands to manipulate various features of the device which would not
> +easily map into the configuration space.

Let's assume device offers both VIRTIO_NET_F_ADMIN_CTRL_VQ and
VIRTIO_NET_F_CTRL_VQ, but driver accepts only VIRTIO_NET_F_CTRL_VQ,
because it is an old driver that only knows about VIRTIO_NET_F_CTRL_VQ.
What happens then? Do we expect the control queue virtqueue to just

I read the proposal like VIRTIO_NET_F_ADMIN_CTRL_VQ and
VIRTIO_NET_F_CTRL_VQ are mutually exclusive features, and not stacking
features, but my confidence in this regard is very low. If they are
mutually exclusive, we should make the feature bits mutually exclusive
as well.

>  All commands are of the following form:
> +is not negotiated:
>  \begin{lstlisting}
>  struct virtio_net_ctrl {
> @@ -3864,6 +3868,29 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  do except issue a diagnostic if \field{ack} is not
> +negotiated, the driver can use the admin control virtqueue of the

There is no such thing defined in the spec like 'admin control
virtqueue'. This is the first and only mention. Do you mean the
controlq (as defined in 5.1.2 a.k.a 'control virtqueue')?

> +management device to manipulate features of individual virtual devices
> +where the control virtqueue is not easily implemented. The definition
> +of management device and virtual device is transport or device
> +specific. 

Is the management device a virtio network device that offered the
VIRTIO_NET_F_ADMIN_CTRL_VQ feature bit? If the answer is yes, does
that device otherwise work like any other virtio network device.

> E.g in the case of PCI SR-IOV, the management device is
> +implemented via the physical function (PF), then the virtual device is
> +the virtual function (VF) in this case.
> +

I'm very superficially acquainted with PCI SR-IOV, maybe that's why I
don't understand everything right away. Please bear with me.

> +All commands are of the following form:
> +
> +\begin{lstlisting}
> +struct virtio_net_admin_ctrl {
> +        u32 virtual_device_id;
> +        struct virtio_net_ctrl ctrl;
> +};
> +\end{lstlisting}
> +
> +The \field{virtual_device_id} is an unique transport or device specific
> +identifier for a virtual device or management device. E.g for the case
> +of PCI SR-IOV, it's the PCI function id. Management device MUST
> +reserve 0 for \field{virtual_device_id} to identify itself.
> +

How is a portable driver supposed to obtain this transport or device
specific ID?

Let me elaborate. Let's say I'm a guest and I happen to have a virtio-net
device, which ain't capable doing the usual controlq stuff via the usual
controlq, so I have to use this new mechanism (if I, for example, want to
set the MAC address for it. To do so, I need to know the
virtual_device_id of my virtual virtio-net device, to be able to put it
in my virtio_net_admin_ctl command, and that command I need to put into
some admin control virtqueue.

The paragraph above suggests, that this virtqueue is not the controlq of
my virtual virtio-net device I'm trying to control, but a virtqueue that
belongs to the management device.

Obviously for a non-PCI device, it ain't making no sense to define this
device id as the PCI function id. I guess, this is where the 'transport
specific' comes from. But then shouldn't we define the transport specific
part in a transport specific section?

Also please notice, that the structure virtio_net_admin_ctrl is defined
in the transport agnostic part, that is it has to work for any
transport. Which implies that any other transport must use ids that fit
32 bits. 

>  \paragraph{Packet Receive Filtering}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Packet Receive Filtering}
>  \label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Setting Promiscuous Mode}%old label for latexdiff
> @@ -4308,6 +4335,32 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  according to the native endian of the guest rather than
>  (necessarily when not using the legacy interface) little-endian.
> +\paragraph{Setting Trust for Virtual Device}\label{sec:Device Types / Network Device / Device Operation / Control Virtqueue / Setting Trust for Virtual Device}
> +
> +If the VIRTIO_NET_F_ADMIN_CTRL_VQ is negotiated, the management device
> +can accept operations via the control vq from the trusted virtual
> +device.
> +
> +\begin{lstlisting}
> +\end{lstlisting}
> +
> +\devicenormative{\subparagraph}{Setting Trust for Virtual Device}{Device Types / Network Device / Device Operation / Control Virtqueue / Setting Trust for Virtual Device}
> +
> +If the VIRTIO_NET_F_ADMIN_CTRL_VQ has been negotiated, the device MUST
> +command. 

What does it mean to support the class? I guess it is still allowed
to post VIRTIO_NET_ERR in the ack field, because the device may be

What should the device do when it encounters a not supported class?

> +control virtqueue of a specific virtual device on and off. The command
> +specific data is one byte containing 0(off) or 1(on).  When trust is
> +off the device MUST fail any operations from the control virtqueue of
> +the virtual device. The management device MUST NOT trust any virtual
> +devices after reset.

This reads like, the famous admin control virtqueue may actually be the
controlq of the device, that one wants to control, if that device was
previously made trusted by a VIRTIO_NET_ADMIN_CTRL_TRUST_ENABLE command.

From the rest of the thread, it appears that we are going to drop the
trust stuff. That makes the question can a trusted virtual device control
another virtual device (by putting not its own id, but the other devices
id into the command) moot.

But it also makes the question how is a guest going to control it's
virtual virtio-net device a more relevant one, because that virtual
device being trusted, and using its controlq is not an option any more.


> +
> +\drivernormative{\subparagraph}{Setting Trust for Virtual Device}{Device Types / Network Device / Device Operation / Control Virtqueue / Setting Trust for Virtual Device}
> +
> +A driver SHOULD negotiate VIRTIO_NET_F_ADMIN_CTRL_VQ if the device
> +offers it.
>  \subsubsection{Legacy Interface: Framing Requirements}\label{sec:Device
>  Types / Network Device / Legacy Interface: Framing Requirements}

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