Block Device VIRTIO_BLK_F_RO clarification needed.

[Damir Shaikhutdinov <Damir.Shaikhutdinov@opensynergy.com>]

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?

With best regards,
Damir Shaikhutdinov

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