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
