9p: Added virtfs option 'multidevs=remap|forbid|warn'

'warn' (default): Only log an error message (once) on host if more than one
device is shared by same export, except of that just ignore this config
error though. This is the default behaviour for not breaking existing
installations implying that they really know what they are doing.

'forbid': Like 'warn', but except of just logging an error this
also denies access of guest to additional devices.

'remap': Allows to share more than one device per export by remapping
inodes from host to guest appropriately. To support multiple devices on the
9p share, and avoid qid path collisions we take the device id as input to
generate a unique QID path. The lowest 48 bits of the path will be set
equal to the file inode, and the top bits will be uniquely assigned based
on the top 16 bits of the inode and the device id.

Signed-off-by: Antonios Motakis <antonios.motakis@huawei.com>
[CS: - Rebased to https://github.com/gkurz/qemu/commits/9p-next
       (SHA1 7fc4c49e91).
     - Added virtfs option 'multidevs', original patch simply did the inode
       remapping without being asked.
     - Updated hash calls to new xxhash API.
     - Updated docs for new option 'multidevs'.
     - Fixed v9fs_do_readdir() not having remapped inodes.
     - Log error message when running out of prefixes in
       qid_path_prefixmap().
     - Fixed definition of QPATH_INO_MASK.
     - Wrapped qpp_table initialization to dedicated qpp_table_init()
       function.
     - Dropped unnecessary parantheses in qpp_lookup_func().
     - Dropped unnecessary g_malloc0() result checks. ]
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
[groug: - Moved "multidevs" parsing to the local backend.
        - Added hint to invalid multidevs option error.
	- Turn "remap" into "x-remap". ]
Signed-off-by: Greg Kurz <groug@kaod.org>

1 files changed, 24 insertions, 2 deletions

diff --git a/qemu-options.hx b/qemu-options.hx
index 2a04ca6ac5..793d70ff93 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -1339,7 +1339,7 @@ ETEXI
 
 DEF("virtfs", HAS_ARG, QEMU_OPTION_virtfs,
     "-virtfs local,path=path,mount_tag=tag,security_model=mapped-xattr|mapped-file|passthrough|none\n"
-    "        [,id=id][,writeout=immediate][,readonly][,fmode=fmode][,dmode=dmode]\n"
+    "        [,id=id][,writeout=immediate][,readonly][,fmode=fmode][,dmode=dmode][,multidevs=remap|forbid|warn]\n"
     "-virtfs proxy,mount_tag=tag,socket=socket[,id=id][,writeout=immediate][,readonly]\n"
     "-virtfs proxy,mount_tag=tag,sock_fd=sock_fd[,id=id][,writeout=immediate][,readonly]\n"
     "-virtfs synth,mount_tag=tag[,id=id][,readonly]\n",
@@ -1347,7 +1347,7 @@ DEF("virtfs", HAS_ARG, QEMU_OPTION_virtfs,
 
 STEXI
 
-@item -virtfs local,path=@var{path},mount_tag=@var{mount_tag} ,security_model=@var{security_model}[,writeout=@var{writeout}][,readonly] [,fmode=@var{fmode}][,dmode=@var{dmode}]
+@item -virtfs local,path=@var{path},mount_tag=@var{mount_tag} ,security_model=@var{security_model}[,writeout=@var{writeout}][,readonly] [,fmode=@var{fmode}][,dmode=@var{dmode}][,multidevs=@var{multidevs}]
 @itemx -virtfs proxy,socket=@var{socket},mount_tag=@var{mount_tag} [,writeout=@var{writeout}][,readonly]
 @itemx -virtfs proxy,sock_fd=@var{sock_fd},mount_tag=@var{mount_tag} [,writeout=@var{writeout}][,readonly]
 @itemx -virtfs synth,mount_tag=@var{mount_tag}
@@ -1403,6 +1403,28 @@ Specifies the default mode for newly created directories on the host. Works
 only with security models "mapped-xattr" and "mapped-file".
 @item mount_tag=@var{mount_tag}
 Specifies the tag name to be used by the guest to mount this export point.
+@item multidevs=@var{multidevs}
+Specifies how to deal with multiple devices being shared with a 9p export.
+Supported behaviours are either "remap", "forbid" or "warn". The latter is
+the default behaviour on which virtfs 9p expects only one device to be
+shared with the same export, and if more than one device is shared and
+accessed via the same 9p export then only a warning message is logged
+(once) by qemu on host side. In order to avoid file ID collisions on guest
+you should either create a separate virtfs export for each device to be
+shared with guests (recommended way) or you might use "remap" instead which
+allows you to share multiple devices with only one export instead, which is
+achieved by remapping the original inode numbers from host to guest in a
+way that would prevent such collisions. Remapping inodes in such use cases
+is required because the original device IDs from host are never passed and
+exposed on guest. Instead all files of an export shared with virtfs always
+share the same device id on guest. So two files with identical inode
+numbers but from actually different devices on host would otherwise cause a
+file ID collision and hence potential misbehaviours on guest. "forbid" on
+the other hand assumes like "warn" that only one device is shared by the
+same export, however it will not only log a warning message but also
+deny access to additional devices on guest. Note though that "forbid" does
+currently not block all possible file access operations (e.g. readdir()
+would still return entries from other devices).
 @end table
 ETEXI