Re: [virtio] [PATCH v5] virtio-blk: restore VIRTIO_BLK_F_FLUSH and VIRTIO_BLK_F_CONFIG_WCE

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

On Tue, Aug 11, 2015 at 03:41:20PM +0200, Cornelia Huck wrote:
> On Tue, 11 Aug 2015 16:23:39 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > OK to summarize, how about:
> > 
> > In free text:
> > 
> > The cache mode after reset can be either writeback or writethrough.
> > The actual mode can be determined by reading \field{writeback}.
> > 
> > In conformance paragraph:
> > 
> > The driver MUST NOT read \field{writeback} before setting
> > the FEATURES_OK \field{status} bit.
> > 
> > 
> > And in legacy section:
> > 
> > Legacy drivers and drivers did not support the FEATURES_OK \field{status} bit,
> > and used the device before setting the DRIVER_OK bit.
> > Therefore:
> > 
> > When using the legacy interface, the driver MAY read \field{writeback}
> > before setting the FEATURES_OK \field{status} field.
> 
> But if it is using the legacy interface, it won't ever set FEATURES_OK,
> no? The wording is a bit confusing.
> 
> "As the legacy interface did not support FEATURES_OK to signal end of
> feature negotation, the driver MAY read \field{writeback} without the
> FEATURES_OK \field{status} bit having been set."
> 
> ?

Right.

> Do we need to mandate that the device MUST assume the driver is operating
> in legacy mode in that case?

Not sure I understand - in what case?

> > When using the legacy interface, the transitional devices MAY assume
> > that driver features won't be changed after reading \field{writeback}.
> 
> "For a driver using the legacy interface, the transitional device MAY
> assume that the driver won't change features after reading
> \field{writeback}."
> 
> ?

Except we say "When using the legacy interface" everywhere,
let's keep it consistent pls.

	When using the legacy interface, the transitional device MAY
	assume that the driver won't change features after reading
	\field{writeback}."

point is both driver and device are using the legacy interface.

-- 
MST