From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-1019-cohuck=redhat.com@lists.oasis-open.org Sender: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 99BF6985F8C for ; Wed, 22 Jan 2020 05:53:44 +0000 (UTC) Date: Wed, 22 Jan 2020 00:53:37 -0500 From: "Michael S. Tsirkin" Message-ID: <20200122005225-mutt-send-email-mst@kernel.org> References: <21D64D2457144C4AA23D071E2F6A51EC01592F5BFC@MXS01.open-synergy.com> <20200120111648-mutt-send-email-mst@kernel.org> <20200121160818.GE641751@stefanha-x1.localdomain> MIME-Version: 1.0 In-Reply-To: <20200121160818.GE641751@stefanha-x1.localdomain> Subject: Re: [virtio-comment] Block Device VIRTIO_BLK_F_RO clarification needed. Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline To: Stefan Hajnoczi Cc: Damir Shaikhutdinov , "virtio-comment@lists.oasis-open.org" List-ID: 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! > > >=20 > > > While reading section 5.2 of the spec, I found that VIRTIO_BLK_F_RO f= eature could use > > > some additional clarification regarding handling requests when it was= offered. > > >=20 > > > 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 desc= ribe a "write request". > > >=20 > > > 5.2.5 p3 says "If the VIRTIO_BLK_F_RO feature is set by the device, a= ny write requests will fail." > > >=20 > > > 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 p= 3 is VIRTIO_BLK_T_OUT. > > >=20 > > > 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." > > >=20 > > > What is missing here is a requirement for features and other types of= requests in case of VIRTIO_BLK_F_RO: > > >=20 > > > * 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 fea= ture) > > >=20 > > > Configuration field, requests and features described above make no se= nse in presence of VIRTIO_BLK_F_RO. > > >=20 > > > This can be clarified in several ways, for example: > > >=20 > > > 1. Device should not offer CONFIG_WCE, FLUSH, DISCARD and WRITE_ZEROE= S features if it offers RO feature (partially contradicts 5.2.5.2 "Device S= HOULD always offer VIRTIO_BLK_F_FLUSH") > > > 2. Device must set the status byte to IOERR for FLUSH, DISCARD and WR= ITE_ZEROES requests if it offers RO feature. > > >=20 > > >=20 > > > What do you think about that? > >=20 > > I guess flush can even be allowed, it's harmless if nothing is written, > > right? >=20 > 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). >=20 > > I agree DISCARD and WRITE_ZEROES must fail. And set VIRTIO_BLK_S_IOERR > > I guess? >=20 > Yes. >=20 > > It's too late to disallow configurations such as a combination of VIRTI= O_BLK_F_CONFIG_WCE > > and VIRTIO_BLK_F_RO, devices out there might be setting this combinatio= n. >=20 > Yes. >=20 > Stefan Thanks! Stefan do you have the time to work on a spec patch to fix this? Damir do you? --=20 MST This publicly archived list offers a means to provide input to the=0D OASIS Virtual I/O Device (VIRTIO) TC.=0D =0D In order to verify user consent to the Feedback License terms and=0D to minimize spam in the list archive, subscription is required=0D before posting.=0D =0D Subscribe: virtio-comment-subscribe@lists.oasis-open.org=0D Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org=0D List help: virtio-comment-help@lists.oasis-open.org=0D List archive: https://lists.oasis-open.org/archives/virtio-comment/=0D Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf= =0D List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lis= ts=0D Committee: https://www.oasis-open.org/committees/virtio/=0D Join OASIS: https://www.oasis-open.org/join/