Re: [virtio-comment] Block Device VIRTIO_BLK_F_RO clarification needed.

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

On Tue, Jan 21, 2020 at 04:08:18PM +0000, Stefan Hajnoczi wrote:
> On Mon, Jan 20, 2020 at 11:21:36AM -0500, Michael S. Tsirkin wrote:
> > On Mon, Jan 20, 2020 at 11:45:05AM +0000, Damir Shaikhutdinov wrote:
> > > Hello!
> > > 
> > > While reading section 5.2 of the spec, I found that VIRTIO_BLK_F_RO feature could use
> > > some additional clarification regarding handling requests when it was offered.
> > > 
> > > This RO feature is described as "Device is read only" (Section 5.2.3). There are some explicit requirements for this feature, but they only describe a "write request".
> > > 
> > > 5.2.5 p3 says "If the VIRTIO_BLK_F_RO feature is set by the device, any write requests will fail."
> > > 
> > > 5.2.6 says "The type of the request is either a read (VIRTIO_BLK_T_IN), a write (VIRTIO_BLK_T_OUT) ...", so "write request" mentioned in 5.2.5 p3 is VIRTIO_BLK_T_OUT.
> > > 
> > > 5.2.6.2 says "A device MUST set the status byte to VIRTIO_BLK_S_IOERR for a write request if the VIRTIO_BLK_F_RO
> > > feature if offered, and MUST NOT write any data."
> > > 
> > > What is missing here is a requirement for features and other types of requests in case of VIRTIO_BLK_F_RO:
> > > 
> > > * config.writeback field (guarded by VIRTIO_BLK_F_CONFIG_WCE feature)
> > > * VIRTIO_BLK_T_ FLUSH (guarded by VIRTIO_BLK_F_FLUSH feature)
> > > * VIRTIO_BLK_T_DISCARD (guarded by VIRTIO_BLK_F_DISCARD feature)
> > > * VIRTIO_BLK_T_WRITE_ZEROES (guarded by VIRTIO_BLK_F_WRITE_ZEROES feature)
> > > 
> > > Configuration field, requests and features described above make no sense in presence of VIRTIO_BLK_F_RO.
> > > 
> > > This can be clarified in several ways, for example:
> > > 
> > > 1. Device should not offer CONFIG_WCE, FLUSH, DISCARD and WRITE_ZEROES features if it offers RO feature (partially contradicts 5.2.5.2 "Device SHOULD always offer VIRTIO_BLK_F_FLUSH")
> > > 2. Device must set the status byte to IOERR for FLUSH, DISCARD and WRITE_ZEROES requests if it offers RO feature.
> > > 
> > > 
> > > What do you think about that?
> > 
> > I guess flush can even be allowed, it's harmless if nothing is written,
> > right?
> 
> I agree.  There seems to be no benefit in failing it and there's a small
> chance that some drivers could get upset if it fails (typically when a
> common code path submits a flush request and expects it to succeed).
> 
> > I agree DISCARD and WRITE_ZEROES must fail. And set VIRTIO_BLK_S_IOERR
> > I guess?
> 
> Yes.
> 
> > It's too late to disallow configurations such as a combination of VIRTIO_BLK_F_CONFIG_WCE
> > and VIRTIO_BLK_F_RO, devices out there might be setting this combination.
> 
> Yes.
> 
> Stefan

Thanks! Stefan do you have the time to work on a spec patch to fix this?
Damir do you?

-- 
MST


This publicly archived list offers a means to provide input to the
OASIS Virtual I/O Device (VIRTIO) TC.

In order to verify user consent to the Feedback License terms and
to minimize spam in the list archive, subscription is required
before posting.

Subscribe: virtio-comment-subscribe@lists.oasis-open.org
Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org
List help: virtio-comment-help@lists.oasis-open.org
List archive: https://lists.oasis-open.org/archives/virtio-comment/
Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf
List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists
Committee: https://www.oasis-open.org/committees/virtio/
Join OASIS: https://www.oasis-open.org/join/