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

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/07/2015 13:21, James Bottomley wrote:
> > +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.
>
> Not SHOULD, MUST.  Please don't leave spec based doors open to bad
> implementations.  An implementation that allows data corruption MUST NOT
> have a spec interpretation that supports it.

What I'm trying to do is to make it possible for the guest to detect an
implementation that is intentionally unsafe.  Such an implementation
MUST NOT propose the VIRTIO_BLK_F_FLUSH feature, unlike the USB drives
you mention.  I totally agree that WCE and FLUSH are not separable.

Paolo