On Mon, Oct 04, 2021 at 02:01:14PM +0200, Cornelia Huck wrote:

On Sun, Oct 03 2021, "Michael S. Tsirkin" [off-list ref] wrote:

quoted

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.

quoted

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?

I guess. OK we can make it MUST.

quoted

+
+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/

?

Both versions are actually confused, since how do you
find out that device does not offer VIRTIO_F_VERSION_1?

I think what this should really say is

Transitional drivers MUST NOT accept VIRTIO_F_VERSION_1 through
the legacy interface.


Does linux actually satisfy this? Will it accept VIRTIO_F_VERSION_1
through the legacy interface if offered?

quoted

+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.

Do we want to also add explanation that features can be
changed until FEATURES_OK?

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