scsi, file-posix: add support for persistent reservation management

It is a common requirement for virtual machine to send persistent
reservations, but this currently requires either running QEMU with
CAP_SYS_RAWIO, or using out-of-tree patches that let an unprivileged
QEMU bypass Linux's filter on SG_IO commands.

As an alternative mechanism, the next patches will introduce a
privileged helper to run persistent reservation commands without
expanding QEMU's attack surface unnecessarily.

The helper is invoked through a "pr-manager" QOM object, to which
file-posix.c passes SG_IO requests for PERSISTENT RESERVE OUT and
PERSISTENT RESERVE IN commands.  For example:

  $ qemu-system-x86_64
      -device virtio-scsi \
      -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
      -drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
      -device scsi-block,drive=hd

or:

  $ qemu-system-x86_64
      -device virtio-scsi \
      -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
      -blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
      -device scsi-block,drive=hd

Multiple pr-manager implementations are conceivable and possible, though
only one is implemented right now.  For example, a pr-manager could:

- talk directly to the multipath daemon from a privileged QEMU
  (i.e. QEMU links to libmpathpersist); this makes reservation work
  properly with multipath, but still requires CAP_SYS_RAWIO

- use the Linux IOC_PR_* ioctls (they require CAP_SYS_ADMIN though)

- more interestingly, implement reservations directly in QEMU
  through file system locks or a shared database (e.g. sqlite)

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/qapi/block-core.json b/qapi/block-core.json
index bb11815..c69a395 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2241,6 +2241,9 @@
 # Driver specific block device options for the file backend.
 #
 # @filename:    path to the image file
+# @pr-manager:  the id for the object that will handle persistent reservations
+#               for this device (default: none, forward the commands via SG_IO;
+#               since 2.11)
 # @aio:         AIO backend (default: threads) (since: 2.8)
 # @locking:     whether to enable file locking. If set to 'auto', only enable
 #               when Open File Descriptor (OFD) locking API is available
@@ -2250,6 +2253,7 @@
 ##
 { 'struct': 'BlockdevOptionsFile',
   'data': { 'filename': 'str',
+            '*pr-manager': 'str',
             '*locking': 'OnOffAuto',
             '*aio': 'BlockdevAioOptions' } }