Re: [RFC PATCH 1/1] virtio: write back features before verify

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Oct 03, 2021 at 02:42:30AM -0400, Michael S. Tsirkin wrote:
> On Sun, Oct 03, 2021 at 07:00:30AM +0200, Halil Pasic wrote:
> > On Sat, 2 Oct 2021 14:20:47 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > > >From my perspective the problem is that the version of the device  
> > > > remains in limbo as long as the features have not yet been finalized,
> > > > which means that the endianness of the config space remains in limbo as
> > > > well. Both device and driver might come to different conclusions.  
> > > 
> > > Version === legacy versus modern?
> > > It is true that feature negotiation can not be used by device to decide that
> > > question simply because it happens too late.
> > > So let's not use it for that then ;)
> > > 
> > > Yes we have VERSION_1 which looks like it should allow this, but
> > > unfortunately it only helps with that for the driver, not the device.
> > > 
> > > In practice legacy versus modern has to be determined by
> > > transport specific versioning, luckily we have that for all
> > > specified transports (can't say what happens with rproc).
> > 
> > So if we look at ccw, you say that the revision negotiation already
> > determines whether VERSION_1 is negotiated or not, and the
> > feature bit VERSION_1 is superfluous?
> > 
> > That would also imply, that 
> > 1) if revision > 0 was negotiated then the device must offer VERSION_1
> > 2) if revision > 0 was negotiated and the driver cleared VERSION_1
> >    the device must refuse to operate.
> > 3) if revision > 0 was negotiated then the driver should reject 
> >    to drive a device if it does not offer VERSION_1
> > 4) if revision > 0 was negotiated the driver must accept VERSION_1
> > 5) if revision > 0 was *not* negotiated then the device should not offer
> >    VERSION_1 because at this point it is already certain that the device
> >    can not act in accordance to the virtio 1.0 or higher interface.
> > 
> > Does that sound about right?
> 
> To me, it does.
> 
> > IMHO we should also change 
> > https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-160003
> > and the definition of VERSION_1 because both sides have to know what is
> > going on before features are fully negotiated. Or?
> > 
> > Regards,
> > Halil
> > 
> 
> I guess so. And I guess we need transport-specific sections
> describing this behaviour for each transport.
> 
> So something like this, for starters?

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