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

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

Hi Stefan and Michael!

I have time to prepare the spec patch and will work on it.

Damir Shaikhutdinov
Senior Software Engineer

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________________________________________
From: Stefan Hajnoczi [stefanha@redhat.com]
Sent: Wednesday, January 22, 2020 11:12 AM
To: Michael S. Tsirkin
Cc: Damir Shaikhutdinov; virtio-comment@lists.oasis-open.org
Subject: Re: [virtio-comment] Block Device VIRTIO_BLK_F_RO clarification needed.

On Wed, Jan 22, 2020 at 12:53:37AM -0500, Michael S. Tsirkin wrote:
> 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?

Damir, you can find instructions for contributing to the virtio
specification here:

https://github.com/oasis-tcs/virtio-spec/

If you don't have time to write a patch please let me know.

Stefan

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