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From 5d77bd2fe5220f026648c9e2f49e65ff45c5475d Mon Sep 17 00:00:00 2001
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From: John Snow <jsnow@redhat.com>
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Date: Tue, 9 Oct 2018 22:14:32 +0200
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Subject: [PATCH 02/22] nbd: fix NBD_FLAG_SEND_CACHE value
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RH-Author: John Snow <jsnow@redhat.com>
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Message-id: <20181009221432.18359-3-jsnow@redhat.com>
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Patchwork-id: 82509
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O-Subject: [RHEL-7.6 qemu-kvm-rhev PATCH 2/2] nbd: fix NBD_FLAG_SEND_CACHE value
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Bugzilla: 1636148
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RH-Acked-by: Eric Blake <eblake@redhat.com>
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RH-Acked-by: Thomas Huth <thuth@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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From: "Denis V. Lunev" <den@openvz.org>
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Commit bc37b06a5 added NBD_CMD_CACHE support, but used the wrong value
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for NBD_FLAG_SEND_CACHE flag for negotiation. That commit picked bit 8,
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which had already been assigned by the NBD specification to mean
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NBD_FLAG_CAN_MULTI_CONN, and which was already implemented in the
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Linux kernel as a part of stable userspace-kernel API since 4.10:
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"bit 8, NBD_FLAG_CAN_MULTI_CONN: Indicates that the server operates
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entirely without cache, or that the cache it uses is shared among all
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connections to the given device. In particular, if this flag is
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present, then the effects of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA
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MUST be visible across all connections when the server sends its reply
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to that command to the client. In the absense of this flag, clients
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SHOULD NOT multiplex their commands over more than one connection to
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the export.
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...
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bit 10, NBD_FLAG_SEND_CACHE: documents that the server understands
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NBD_CMD_CACHE; however, note that server implementations exist
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which support the command without advertising this bit, and
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conversely that this bit does not guarantee that the command will
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succeed or have an impact."
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Consequences:
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- a client trying to use NBD_CMD_CACHE per the NBD spec will not
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see the feature as available from a qemu 3.0 server (not fatal,
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clients already have to be prepared for caching to not exist)
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- a client accidentally coded to the qemu 3.0 bit value instead
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of following the spec may interpret NBD_CMD_CACHE as being available
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when it is not (probably not fatal, the spec says the server should
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gracefully fail unknown commands, and that clients of NBD_CMD_CACHE
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should be prepared for failure even when the feature is advertised);
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such clients are unlikely (perhaps only in unreleased Virtuozzo code),
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and will disappear over time
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- a client prepared to use multiple connections based on
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NBD_FLAG_CAN_MULTI_CONN may cause data corruption when it assumes
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that caching is consistent when in reality qemu 3.0 did not have
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a consistent cache. Partially mitigated by using read-only
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connections (where nothing needs to be flushed, so caching is
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indeed consistent) or when using qemu-nbd with the default -e 1
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(at most one client at a time); visible only when using -e 2 or
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more for a writable export.
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Thus the commit fixes negotiation flag in QEMU according to the
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specification.
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Signed-off-by: Denis V. Lunev <den@openvz.org>
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CC: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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CC: Valery Vdovin <valery.vdovin@acronis.com>
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CC: Eric Blake <eblake@redhat.com>
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CC: Paolo Bonzini <pbonzini@redhat.com>
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CC: qemu-stable@nongnu.org
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Message-Id: <20181004100313.4253-1-den@openvz.org>
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Reviewed-by: Eric Blake <eblake@redhat.com>
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[eblake: enhance commit message, add defines for unimplemented flags]
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Signed-off-by: Eric Blake <eblake@redhat.com>
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(cherry picked from commit df91328adab8490367776d2b21b35d790a606120)
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Signed-off-by: John Snow <jsnow@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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include/block/nbd.h | 4 +++-
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1 file changed, 3 insertions(+), 1 deletion(-)
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diff --git a/include/block/nbd.h b/include/block/nbd.h
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index 4638c83..a53b0cf 100644
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--- a/include/block/nbd.h
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+++ b/include/block/nbd.h
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@@ -135,7 +135,9 @@ typedef struct NBDExtent {
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#define NBD_FLAG_SEND_TRIM (1 << 5) /* Send TRIM (discard) */
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#define NBD_FLAG_SEND_WRITE_ZEROES (1 << 6) /* Send WRITE_ZEROES */
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#define NBD_FLAG_SEND_DF (1 << 7) /* Send DF (Do not Fragment) */
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-#define NBD_FLAG_SEND_CACHE (1 << 8) /* Send CACHE (prefetch) */
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+#define NBD_FLAG_CAN_MULTI_CONN (1 << 8) /* Multi-client cache consistent */
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+#define NBD_FLAG_SEND_RESIZE (1 << 9) /* Send resize */
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+#define NBD_FLAG_SEND_CACHE (1 << 10) /* Send CACHE (prefetch) */
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/* New-style handshake (global) flags, sent from server to client, and
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control what will happen during handshake phase. */
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--
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1.8.3.1
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