Re: [virtio-comment] comment on pci section

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Wed, 05 Feb 2014 09:49:18 +1030
Rusty Russell <rusty@au1.ibm.com> wrote:

> "Michael S. Tsirkin" <mst@redhat.com> writes:
> > On Tue, Feb 04, 2014 at 01:35:55PM +1030, Rusty Russell wrote:
> >> "Michael S. Tsirkin" <mst@redhat.com> writes:
> >> 2) Add the concept of a "live" virtqueue, and explicitly disallow
> >>    moving it backwards or changing descriptors.  Make it clear that
> >>    taking the net device out of mq mode makes the extra vq's not-live.
> >
> > This last part doesn't apply anymore, does it?
> 
> Yes, indeed.
> 
> Here's what I'm proposing to apply, if noone objects:
> 
> Subject: Cleanup and setup clarifications
> From: Rusty Russell <rusty@au1.ibm.com>
> 
> 1) Explicitly allow drivers to read config space during feature
>    negotiation.
> 
> 2) Add the concept of a "live" virtqueue, and explicitly disallow
>    moving it backwards or changing descriptors.
> 
> Signed-off-by: Rusty Russell <rusty@au1.ibm.com>
> 
> diff --git a/content.tex b/content.tex
> index fa33435..413a4ad 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -524,7 +524,11 @@ The driver MUST follow this sequence to initialize a device:
>  \item Set the DRIVER status bit: the guest OS knows how to drive the device.
> 
>  \item Read device feature bits, and write the subset of feature bits
> -   understood by the OS and driver to the device.
> +   understood by the OS and driver to the device.  During this step the
> +   driver MAY read (but MUST NOT write) the device-specific configuration fields to check that it can support the device before accepting it.
> +   The device MUST allow reading of any device-specific configuration field
> +   before FEATURES_OK is set.  This includes fields which are conditional
> +   on feature bits, as long as those feature bits are offered by the device.
> 
>  \item\label{itm:General Initialization And Device Operation / Device Initialization / Set FEATURES-OK} Set the FEATURES_OK status bit.  The driver MUST not accept
>     new feature bits after this step.
> @@ -544,9 +548,9 @@ The driver MUST follow this sequence to initialize a device:
>  If any of these steps go irrecoverably wrong, the driver SHOULD
>  set the FAILED status bit to indicate that it has given up on the
>  device (it can reset the device later to restart if desired).  The
> -driver MUST not continue initialization in that case.
> +driver MUST NOT continue initialization in that case.
> 
> -The device MUST NOT consume buffers before DRIVER_OK, and the driver
> +The device MUST NOT consume buffers or notify the driver before DRIVER_OK, and the driver
>  MUST NOT notify the device before it sets DRIVER_OK.
> 
>  Devices SHOULD support all valid combinations of features, but we know
> @@ -769,7 +773,18 @@ suppressed by the device:
>  For devices where the configuration information can be changed, an
>  interrupt is delivered when a configuration change occurs.
> 
> +\section{Device Cleanup}\label{sec:General Initialization And Device Operation / Device Cleanup}
> 
> +Once the driver has set the DRIVER_OK status bit, all the configured virtqueue of the device are considered live.  All of a device's virtqueues are no longer live once the device has been reset.

What about something similar to my earlier suggestions?

"Once the driver has set the DRIVER_OK status bit, all of the
configured virtqueues of the device are considered live.  Any of the
virtqueues of a device are no longer live once the device has been
reset."

?

> +
> +A driver MUST NOT alter descriptor table entries which have been
> +exposed in the available ring (and not marked consumed by the device
> +in the used ring) of a live virtqueue.
> +
> +A driver MUST NOT decrement the available index on a live virtqueue (ie.
> +do not try to "unexpose" buffers).

"(i.e. the driver MUST NOT try to "unexpose" buffers)."

?

> +
> +Thus a driver MUST ensure a virtqueue isn't live (by device reset) before removing exposed buffers.
> 
>  \chapter{Virtio Transport Options}\label{sec:Virtio Transport Options}