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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>
52 lines
2.4 KiB
ReStructuredText
52 lines
2.4 KiB
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======================================
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Persistent reservation managers
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======================================
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SCSI persistent Reservations allow restricting access to block devices
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to specific initiators in a shared storage setup. When implementing
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clustering of virtual machines, it is a common requirement for virtual
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machines to send persistent reservation SCSI commands. However,
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the operating system restricts sending these commands to unprivileged
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programs because incorrect usage can disrupt regular operation of the
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storage fabric.
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For this reason, QEMU's SCSI passthrough devices, ``scsi-block``
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and ``scsi-generic`` (both are only available on Linux) can delegate
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implementation of persistent reservations to a separate object,
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the "persistent reservation manager". Only PERSISTENT RESERVE OUT and
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PERSISTENT RESERVE IN commands are passed to the persistent reservation
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manager object; other commands are processed by QEMU as usual.
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-----------------------------------------
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Defining a persistent reservation manager
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-----------------------------------------
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A persistent reservation manager is an instance of a subclass of the
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"pr-manager" QOM class.
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Right now only one subclass is defined, ``pr-manager-helper``, which
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forwards the commands to an external privileged helper program
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over Unix sockets. The helper program only allows sending persistent
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reservation commands to devices for which QEMU has a file descriptor,
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so that QEMU will not be able to effect persistent reservations
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unless it has access to both the socket and the device.
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``pr-manager-helper`` has a single string property, ``path``, which
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accepts the path to the helper program's Unix socket. For example,
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the following command line defines a ``pr-manager-helper`` object and
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attaches it to a SCSI passthrough device::
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$ qemu-system-x86_64
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-device virtio-scsi \
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-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
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-drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
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-device scsi-block,drive=hd
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Alternatively, using ``-blockdev``::
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$ qemu-system-x86_64
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-device virtio-scsi \
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-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
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-blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
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-device scsi-block,drive=hd
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