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[Qemu-devel] [PATCH 56/97] nbd: fix NBD_FLAG_SEND_CACHE value
From: |
Michael Roth |
Subject: |
[Qemu-devel] [PATCH 56/97] nbd: fix NBD_FLAG_SEND_CACHE value |
Date: |
Mon, 1 Apr 2019 15:59:30 -0500 |
From: "Denis V. Lunev" <address@hidden>
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 <address@hidden>
CC: Vladimir Sementsov-Ogievskiy <address@hidden>
CC: Valery Vdovin <address@hidden>
CC: Eric Blake <address@hidden>
CC: Paolo Bonzini <address@hidden>
CC: address@hidden
Message-Id: <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
[eblake: enhance commit message, add defines for unimplemented flags]
Signed-off-by: Eric Blake <address@hidden>
(cherry picked from commit df91328adab8490367776d2b21b35d790a606120)
Signed-off-by: Michael Roth <address@hidden>
---
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 4638c839f5..a53b0cfae0 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. */
--
2.17.1
- [Qemu-devel] [PATCH 38/97] nbd/server: fix bitmap export, (continued)
- [Qemu-devel] [PATCH 38/97] nbd/server: fix bitmap export, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 39/97] clean up callback when del virtqueue, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 43/97] block/rbd: add deprecation documentation for filename keyvalue pairs, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 42/97] block/rbd: add iotest for rbd legacy keyvalue filename parsing, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 45/97] ne2000: fix possible out of bound access in ne2000_receive, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 47/97] pcnet: fix possible buffer overflow, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 52/97] target/arm: Correct condition for v8M callee stack push, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 46/97] rtl8139: fix possible out of bound access, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 53/97] nbd/server: fix NBD_CMD_CACHE, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 54/97] intel_iommu: introduce vtd_reset_caches(), Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 56/97] nbd: fix NBD_FLAG_SEND_CACHE value,
Michael Roth <=
- [Qemu-devel] [PATCH 57/97] migration: Stop postcopy fault thread before notifying, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 55/97] intel_iommu: better handling of dmar state switch, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 61/97] net: drop too large packet early, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 65/97] fmops: fix off-by-one in AR_TABLE and DR_TABLE array size, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 66/97] vfio-helpers: Fix qemu_vfio_open_pci() crash, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 67/97] i2c: Move typedef of bitbang_i2c_interface to i2c.h, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 70/97] nbd/client: Make x-dirty-bitmap more reliable, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 72/97] mirror: fix dead-lock, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 73/97] iotests: simple mirror test with kvm on 1G image, Michael Roth, 2019/04/01
- [Qemu-devel] [PATCH 68/97] i2c: Add a length check to the SMBus write handling, Michael Roth, 2019/04/01