Re: [virtio-dev] [PATCH v5 03/10] vhost-user: add requirements for the notification capability

[Stefan Hajnoczi <stefanha@redhat.com>]

On Mon, May 18, 2020 at 11:37:14PM +0300, Nikos Dragazis wrote:
> The configuration structure for the notification capability has certain
> device/driver requirements, similar to those of the MSI-X vector
> configuration. Add these requirements to the device spec.
> 
> Signed-off-by: Nikos Dragazis <ndragazis@arrikto.com>
> ---
>  virtio-vhost-user.tex | 42 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/virtio-vhost-user.tex b/virtio-vhost-user.tex
> index 13ecd41..7a8cd9c 100644
> --- a/virtio-vhost-user.tex
> +++ b/virtio-vhost-user.tex
> @@ -275,6 +275,48 @@ \subsubsection{Notification structure layout}\label{sec:Device Types / Vhost-use
>  \field{VIRTIO_MSI_NO_VECTOR} to \field{notification_msix_vector} to change the
>  MSI-X vector for that notification.
>  
> +\devicenormative{\paragraph}{Notification capability}{Device Types / Vhost-user Device Backend / Additional Device Resources over PCI / Notification capability}
> +
> +If MSI-X is available, device MUST support mapping any master queue
> +event to any valid vector 0 to MSI-X \field{Table Size}. Here

This section combines two things:
1. vhost-user concepts -> VIRTIO notifications
2. VIRTIO notifications -> VIRTIO PCI transport MSI-X implementation of
   notifications

I think this should be split:
1. The virtio-vhost-user device spec should explain that vhost-user queue
   events are mapped to VIRTIO notifications
2. The PCI transport spec should explain how notifications are mapped to
   MSI-X

This way other VIRTIO device types will be able to use notifications too
without everyone explaining MSI-X specifics. Also it won't tie
virtio-vhost-user to VIRTIO PCI.

> +\field{Table Size} is the \field{N-1} encoded Table Size stored in the
> +Message Control register of the MSI-X capability structure according to
> +\hyperref[intro:PCI]{[PCI]}, where \field{N} is the actual MSI-X Table
> +Size.
> +
> +Device MUST support unmapping any master queue event.
> +
> +The device MUST return vector mapped to a given master queue event,
> +(\field{NO_VECTOR} if unmapped) on read of
> +\field{notification_msix_vector}. The device MUST have all master queue
> +events unmapped upon reset.

"The device MUST unmap all master queue events upon reset" is clearer in
indicating that the device must take this action.

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