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From:   Cornelia Huck <cohuck@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Halil Pasic <pasic@linux.ibm.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        Xie Yongji <xieyongji@bytedance.com>,
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        Christian Borntraeger <borntraeger@de.ibm.com>,
        linux-s390@vger.kernel.org, virtio-dev@lists.oasis-open.org
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify
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Date:   Mon, 04 Oct 2021 14:01:14 +0200
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On Sun, Oct 03 2021, "Michael S. Tsirkin" <mst@redhat.com> wrote:

> Sent too early. So here's what I propose. Could you pls take a look
> and if you like this, post a ccw section?

We have not talked about the mmio transport so far, but I guess it
should be fine as legacy and standard are separated.

> There's also an attempt to prevent fallback from modern to legacy
> here since if driver does fallback then failing FEATURES_OK can't work
> properly.
> That's a separate issue, will be a separate patch when I post
> this for consideration by the TC.
>
>
> diff --git a/content.tex b/content.tex
> index 1398390..06271f4 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -140,10 +140,13 @@ \subsection{Legacy Interface: A Note on Feature
>  Bits}\label{sec:Basic Facilities of a Virtio Device / Feature
>  Bits / Legacy Interface: A Note on Feature Bits}
>  
> -Transitional Drivers MUST detect Legacy Devices by detecting that
> -the feature bit VIRTIO_F_VERSION_1 is not offered.
> -Transitional devices MUST detect Legacy drivers by detecting that
> -VIRTIO_F_VERSION_1 has not been acknowledged by the driver.
> +Transitional drivers MAY support operating legacy devices.
> +Transitional devices MAY support operation by legacy drivers.

Why 'MAY'? Isn't the whole point of transitional that it can deal with
both?

> +
> +Transitional drivers MUST detect legacy devices in a way that is
> +transport specific.
> +Transitional devices MUST detect legacy drivers in a way that
> +is transport specific.
>  
>  In this case device is used through the legacy interface.
>  
> @@ -160,6 +163,33 @@ \subsection{Legacy Interface: A Note on Feature
>  Specification text within these sections generally does not apply
>  to non-transitional devices.
>  
> +\begin{note}
> +The device offers different features when used through
> +the legacy interface and when operated in accordance with this
> +specification.
> +\end{note}
> +
> +Transitional drivers MUST use Devices only through the legacy interface

s/Devices only through the legacy interface/devices through the legacy
interface only/

?

> +if the feature bit VIRTIO_F_VERSION_1 is not offered.
> +Transitional devices MUST NOT offer VIRTIO_F_VERSION_1 when used through
> +the legacy interface.
> +
> +When the driver uses a device through the legacy interface, then it
> +MUST only accept the features the device offered through the
> +legacy interface.
> +
> +When used through the legacy interface, the device SHOULD
> +validate that the driver only accepted the features it
> +offered through the legacy interface.
> +
> +When operating a transitional device, a transitional driver
> +SHOULD NOT use the device through the legacy interface if
> +operation through the modern interface has failed.
> +In particular, a transitional driver
> +SHOULD NOT fall back to using the device through the
> +legacy interface if feature negotiation failed
> +(since that would defeat the purpose of the FEATURES_OK bit).
> +
>  \section{Notifications}\label{sec:Basic Facilities of a Virtio Device
>  / Notifications}
>  
> @@ -1003,6 +1033,12 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
>  
>  The driver MUST NOT write a 0 to \field{queue_enable}.
>  
> +\paragraph}{Legacy Interface: Common configuration structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Legacy Interface: Common configuration structure layout}
> +Transitional drivers SHOULD detect legacy devices by detecting
> +that the device has the Transitional PCI Device ID in
> +the range 0x1000 to 0x103f and lacks a VIRTIO_PCI_CAP_COMMON_CFG
> +capability specifying the location of a common configuration structure.
> +
>  \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
>  
>  The notification location is found using the VIRTIO_PCI_CAP_NOTIFY_CFG
> @@ -1288,6 +1324,10 @@ \subsubsection{Legacy Interfaces: A Note on PCI Device Layout}\label{sec:Virtio
>  Transitional devices MUST present part of configuration
>  registers in a legacy configuration structure in BAR0 in the first I/O
>  region of the PCI device, as documented below.
> +
> +Transitional devices SHOULD detect legacy drivers by detecting
> +access to the legacy configuration structure.
> +
>  When using the legacy interface, transitional drivers
>  MUST use the legacy configuration structure in BAR0 in the first
>  I/O region of the PCI device, as documented below.

Generally, looks good to me.