From nobody Wed Oct 29 17:11:30 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) by mx.zohomail.com with SMTPS id 1525269920131445.2916915187087; Wed, 2 May 2018 07:05:20 -0700 (PDT) Received: from localhost ([::1]:50668 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fDsNc-000803-RL for importer@patchew.org; Wed, 02 May 2018 10:05:12 -0400 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50510) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fDsMh-0007OA-Ky for qemu-devel@nongnu.org; Wed, 02 May 2018 10:04:18 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fDsMb-00079y-Io for qemu-devel@nongnu.org; Wed, 02 May 2018 10:04:15 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:60928 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fDsMT-00075H-BV; Wed, 02 May 2018 10:04:01 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id ED513722F4; Wed, 2 May 2018 14:04:00 +0000 (UTC) Received: from localhost (unknown [10.40.205.26]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9763B2026DFD; Wed, 2 May 2018 14:04:00 +0000 (UTC) From: Max Reitz To: qemu-block@nongnu.org Date: Wed, 2 May 2018 16:03:59 +0200 Message-Id: <20180502140359.18222-1-mreitz@redhat.com> X-Scanned-By: MIMEDefang 2.78 on 10.11.54.4 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Wed, 02 May 2018 14:04:00 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.2]); Wed, 02 May 2018 14:04:00 +0000 (UTC) for IP:'10.11.54.4' DOMAIN:'int-mx04.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'mreitz@redhat.com' RCPT:'' X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 66.187.233.73 Subject: [Qemu-devel] [PATCH] block: Document BDRV_REQ_WRITE_UNCHANGED support X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , qemu-devel@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Add BDRV_REQ_WRITE_UNCHANGED to the list of flags honored during pwrite and pwrite_zeroes, and also add a note on when you absolutely need to support it. Signed-off-by: Max Reitz Reviewed-by: Eric Blake --- I did not include a note on how this might be useful to protocol drivers, because BDRV_REQ_WRITE_UNCHANGED requests will usually not end up on the protocol level anyway. (I suppose it would be possible in theory if a protocol driver reports a certain range as unallocated and the user installs a copy-on-read driver on top, but I think every protocol driver currently reports everything as allocated.) --- include/block/block_int.h | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/include/block/block_int.h b/include/block/block_int.h index e3d6219f4e..76b589da57 100644 --- a/include/block/block_int.h +++ b/include/block/block_int.h @@ -656,10 +656,24 @@ struct BlockDriverState { /* I/O Limits */ BlockLimits bl; =20 - /* Flags honored during pwrite (so far: BDRV_REQ_FUA) */ + /* Flags honored during pwrite (so far: BDRV_REQ_FUA, + * BDRV_REQ_WRITE_UNCHANGED). + * If a driver does not support BDRV_REQ_WRITE_UNCHANGED, those + * writes will be issued as normal writes without the flag set. + * This is important to note for drivers that do not explicitly + * request a WRITE permission for their children and instead take + * the same permissions as their parent did (this is commonly what + * block filters do). Such drivers have to be aware that the + * parent may have taken a WRITE_UNCHANGED permission only and is + * issuing such requests. Drivers either must make sure that + * these requests do not result in plain WRITE accesses (usually + * by supporting BDRV_REQ_WRITE_UNCHANGED, and then forwarding + * every incoming write request as-is, including potentially that + * flag), or they have to explicitly take the WRITE permission for + * their children. */ unsigned int supported_write_flags; /* Flags honored during pwrite_zeroes (so far: BDRV_REQ_FUA, - * BDRV_REQ_MAY_UNMAP) */ + * BDRV_REQ_MAY_UNMAP, BDRV_REQ_WRITE_UNCHANGED) */ unsigned int supported_zero_flags; =20 /* the following member gives a name to every node on the bs graph. */ --=20 2.14.3