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