docs/system/pr-manager.rst: Fix minor docs nits

Fix a couple of nits in pr-manager.rst:
* the title marker for the top level heading is overlength
* stray capital 'R' in the middle of a sentence

docs/system/pr-manager.rst: Fix minor docs nits

Fix a couple of nits in pr-manager.rst:
* the title marker for the top level heading is overlength
* stray capital 'R' in the middle of a sentence

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

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docs: Split qemu-pr-helper documentation into tools manual

Split the documentation of the qemu-pr-helper binary into the tools
manual, and give it a manpage like our other standalone executables.

S

docs: Split qemu-pr-helper documentation into tools manual

Split the documentation of the qemu-pr-helper binary into the tools
manual, and give it a manpage like our other standalone executables.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

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docs: Move pr-manager.rst into the system manual

Move the pr-manager documentation into the system manual.
Some of it (the documentation of the pr-manager-helper tool)
should be in tools, but we wil

docs: Move pr-manager.rst into the system manual

Move the pr-manager documentation into the system manual.
Some of it (the documentation of the pr-manager-helper tool)
should be in tools, but we will split it up after moving it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

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scsi: add multipath support to qemu-pr-helper

Proper support of persistent reservation for multipath devices requires
communication with the multipath daemon, so that the reservation is
registered a

scsi: add multipath support to qemu-pr-helper

Proper support of persistent reservation for multipath devices requires
communication with the multipath daemon, so that the reservation is
registered and applied when a path comes up. The device mapper
utilities provide a library to do so; this patch makes qemu-pr-helper.c
detect multipath devices and, when one is found, delegate the operation
to libmpathpersist.

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

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scsi: build qemu-pr-helper

Introduce a privileged helper to run persistent reservation commands.
This lets virtual machines send persistent reservations without using
CAP_SYS_RAWIO or out-of-tree pa

scsi: build qemu-pr-helper

Introduce a privileged helper to run persistent reservation commands.
This lets virtual machines send persistent reservations without using
CAP_SYS_RAWIO or out-of-tree patches. The helper uses Unix permissions
and SCM_RIGHTS to restrict access to processes that can access its socket
and prove that they have an open file descriptor for a raw SCSI device.

The next patch will also correct the usage of persistent reservations
with multipath devices.

It would also be possible to support for Linux's IOC_PR_* ioctls in
the future, to support NVMe devices. For now, however, only SCSI is
supported.

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

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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 wi

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>

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