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

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/08/2015 07:59, Michael S. Tsirkin wrote:
> > +\item If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the cache
> > +  mode can be read or set through the \field{writeback} field.  0 corresponds
> > +  to a writethrough cache, 1 to a writeback cache\footnote{Consistent with
> > +    \ref{devicenormative:Device Types / Block Device / Device Operation},
> > +    a writethrough cache can be defined broadly as a cache that commits
> > +    writes to persistent device backend storage before reporting their
> > +    completion. For example, a battery-backed writeback cache actually
> > +    counts as writethrough according to this definition.}.  The cache mode
> > +  after reset is undefined.
> 
> Is it really undefined, or can it be either
> writeback or writethrough, and can be determined by reading the
> \field{writeback} field reset value?

For transitional devices, it can be either writeback or writethrough and
can be determined by reading \field{writeback}.

For non-transitional devices, it can be determined after you've written
FEATURES_OK.  Until then, you shouldn't read the field.

Paolo