Re: [virtio] [PATCH] virtio-blk: restore VIRTIO_BLK_F_CONFIG_WCE (VIRTIO-144)

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/07/2015 10:48, Michael S. Tsirkin wrote:
>> > 
>> > The above footnote is a consequence of the conformance statements in the
>> > "Device Operation" section.  (Hence putting it in a footnote only).
> Hmm I don't get it.
> 
> The only non-legacy text that mention writethrough is this:
> 	VIRTIO_BLK_F_CONFIG_WCE (11) Device can toggle its cache between
> 	writeback and writethrough modes.
> 
> so this is the only definition of these terms.

After the patch (quoting from v2):

+If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated and the
+\field{writeback} field in configuration space is 0, the device MUST
+ensure that all writes are committed to non-volatile storage before
+reporting completion.
+
+If the VIRTIO_BLK_F_CONFIG_WCE feature is negotiated, the driver MAY
+switch to writethrough or writeback mode by writing respectively 0 and
+1 to the \field{writeback} field.  After writing a 0 to \field{writeback},
+the driver MUST NOT assume that writes submitted while the field was 1 have
+been committed to non-volatile storage.

> Also, isn't the flush command special in the writeback mode?
> maybe mention this...

Again quoting from v2:

+If the \field{VIRTIO_BLK_F_FLUSH} feature is negotiated, upon receipt
+of a VIRTIO_BLK_T_FLUSH request, the device MUST ensure that any writes
+which were completed are committed to non-volatile storage.

> 
> Also
> 	Some older legacy devices did not operate in writethrough mode
> 	even after a driver announced lack of
> 	support for VIRTIO_BLK_F_FLUSH.
> 
> What does this mean? let's just say in which mode they did operate.
> Also what should driver do in this case?

In v2:

+If the \field{VIRTIO_BLK_F_FLUSH} feature is not negotiated, the
+device SHOULD ensure that all writes are committed to non-volatile
+storage before reporting completion.  It MUST do so if it proposed
+\field{VIRTIO_BLK_F_FLUSH}.  Failure to do so can cause data loss.

(No humor intended, I know this is not your field of expertise).

Paolo