From patchwork Tue Apr 2 16:58:03 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Paul Durrant X-Patchwork-Id: 10882135 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id A58781669 for ; Tue, 2 Apr 2019 17:00:28 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 7B1C9288F7 for ; Tue, 2 Apr 2019 17:00:28 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 7917E2891A; Tue, 2 Apr 2019 17:00:28 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=2.0 tests=BAYES_00,MAILING_LIST_MULTI, RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 8C3CB288F7 for ; Tue, 2 Apr 2019 17:00:27 +0000 (UTC) Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1hBMjp-0007g0-68; Tue, 02 Apr 2019 16:58:17 +0000 Received: from us1-rack-dfw2.inumbo.com ([104.130.134.6]) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1hBMjn-0007fv-Nu for xen-devel@lists.xenproject.org; Tue, 02 Apr 2019 16:58:15 +0000 X-Inumbo-ID: 8091471e-5568-11e9-bc90-bc764e045a96 Received: from SMTP03.CITRIX.COM (unknown [162.221.156.55]) by us1-rack-dfw2.inumbo.com (Halon) with ESMTPS id 8091471e-5568-11e9-bc90-bc764e045a96; Tue, 02 Apr 2019 16:58:13 +0000 (UTC) X-IronPort-AV: E=Sophos;i="5.60,301,1549929600"; d="scan'208";a="82526287" From: Paul Durrant To: Date: Tue, 2 Apr 2019 17:58:03 +0100 Message-ID: <20190402165803.28667-1-paul.durrant@citrix.com> X-Mailer: git-send-email 2.20.1.2.gb21ebb6 MIME-Version: 1.0 Subject: [Xen-devel] [PATCH v2] public/io/blkif.h: try to fix the semantics of sector based quantities X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Stefano Stabellini , Wei Liu , Konrad Rzeszutek Wilk , George Dunlap , Andrew Cooper , Ian Jackson , Tim Deegan , Julien Grall , Paul Durrant , Jan Beulich , Athony PERARD , =?utf-8?q?Roger_Pau_Monn=C3=A9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" X-Virus-Scanned: ClamAV using ClamSMTP This is actually v2 of the patch posted in [1]. Please see the thread starting there for more context... The semantics of sector based quanties, such as first_sect and last_sect in blkif_request_segment, and the value of "sectors" in the backend info in xenstore have become confused. Some comments in the header suggest they should be supplied/interpreted strictly in terms of 512-byte units, others suggest they should be scaled by the value of "sector-size" i.e. the logical block size of the underlying backend storage. This confusion has caused mixed semantics to become ingrained in frontend implementations. For instance Linux xen-blkfront.c contains code such as: fsect = offset >> 9; lsect = fsect + (len >> 9) - 1; whereas the Windows XENVBD frontend contains the following equivalent code: Segment->FirstSector = (UCHAR)((Offset + SectorSize - 1) / SectorSize); *SectorsNow = __min(SectorsLeft, SectorsPerPage - Segment->FirstSector); Segment->LastSector = (UCHAR)(Segment->FirstSector + *SectorsNow - 1); (where SectorSize is the "sector-size" value advertized in xenstore). Thus it has become unsafe for a backend to set "sector-size" to anything other than 512 as it does not know which way the frontend is coded. This patch is intended to clarify the situation and also introduce a mechanism to allow logical block sizes of more than 512 to be supported... A new frontend feature node is specified: 'feature-large-sector-size'. If this node is present and set to "1" then it means that frontend is coded to supply and interpret all sector based quantities in terms of the the advertized "sector-size" value rather than a hardcoded size of 512. [1] https://lists.xenproject.org/archives/html/xen-devel/2019-03/msg01600.html Signed-off-by: Paul Durrant Reviewed-by: Juergen Gross --- Cc: Konrad Rzeszutek Wilk Cc: Roger Pau Monné Cc: Athony PERARD Cc: Andrew Cooper Cc: George Dunlap Cc: Ian Jackson Cc: Jan Beulich Cc: Julien Grall Cc: Stefano Stabellini Cc: Tim Deegan Cc: Wei Liu v2: - Drop Konrad's original ack since the patch has substantially changed - Cc several different OS blkfront maintainers and 'the rest' maintainers for more opinion --- xen/include/public/io/blkif.h | 40 ++++++++++++++++++++++++++--------- 1 file changed, 30 insertions(+), 10 deletions(-) diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h index 15a71e3fea..d4a34de94c 100644 --- a/xen/include/public/io/blkif.h +++ b/xen/include/public/io/blkif.h @@ -254,18 +254,26 @@ * sector-size * Values: * - * The logical sector size, in bytes, of the backend device. + * The logical block size, in bytes, of the underlying storage. This + * must be a power of two with a minimum value of 512. + * + * NOTE: Because of implementation bugs in some frontends this must be + * set to 512, unless the frontend advertizes a non-zero value + * in its "feature-large-sector-size" xenbus node. (See below). * * physical-sector-size * Values: + * Default Value: <"sector-size"> * - * The physical sector size, in bytes, of the backend device. + * The physical block size, in bytes, of the backend storage. This + * must be an integer multiple of "sector-size". * * sectors * Values: * - * The size of the backend device, expressed in units of its logical - * sector size ("sector-size"). + * The size of the backend device, expressed in units of "sector-size". + * The product of "sector-size" and "sectors" must also be an integer + * multiple of "physical-sector-size", if that node is present. * ***************************************************************************** * Frontend XenBus Nodes @@ -321,6 +329,8 @@ * The size of the frontend allocated request ring buffer in units of * machine pages. The value must be a power of 2. * + *--------------------------------- Features --------------------------------- + * * feature-persistent * Values: 0/1 (boolean) * Default Value: 0 @@ -342,6 +352,17 @@ * decides to limit the maximum number of persistently mapped grants * to a value less than RING_SIZE * BLKIF_MAX_SEGMENTS_PER_REQUEST. * + * feature-large-sector-size + * Values: 0/1 (boolean) + * Default Value: 0 + * + * A value of "1" indicates that the frontend will correctly supply and + * interpret all sector-based quantities in terms of the "sector-size" + * value supplied in the backend info, whatever that may be set to. + * If this node is not present or its value is "0" then it is assumed + * that the frontend requires that the logical block size is 512 as it + * is hardcoded (which is the case in some frontend implementations). + * *------------------------- Virtual Device Properties ------------------------- * * device-type @@ -607,12 +628,11 @@ #define BLKIF_MAX_INDIRECT_PAGES_PER_REQUEST 8 /* - * NB. first_sect and last_sect in blkif_request_segment, as well as - * sector_number in blkif_request, are always expressed in 512-byte units. - * However they must be properly aligned to the real sector size of the - * physical disk, which is reported in the "physical-sector-size" node in - * the backend xenbus info. Also the xenbus "sectors" node is expressed in - * 512-byte units. + * NB. 'first_sect' and 'last_sect' in blkif_request_segment, as well as + * 'sector_number' in blkif_request, blkif_request_discard and + * blkif_request_indirect are sector-based quantities. See the description + * of the "feature-large-sector-size" frontend xenbus node above for + * more information. */ struct blkif_request_segment { grant_ref_t gref; /* reference to I/O buffer frame */