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qemu-options: finesse the recommendations around -blockdev

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We are a bit premature in recommending -blockdev/-device as the best
way to configure block devices. It seems there are times the more
human friendly -drive still makes sense especially when -snapshot is
involved.

Improve the language to hopefully make things clearer.

Suggested-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230424092249.58552-7-alex.bennee@linaro.org>
This commit is contained in:
Alex Bennée 2023-04-24 10:22:37 +01:00
parent 8b869aa591
commit c1654c3e37
1 changed files with 22 additions and 2 deletions
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24
qemu-options.hx
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@ -1143,10 +1143,22 @@ have gone through several iterations as the feature set and complexity
of the block layer have grown. Many online guides to QEMU often
reference older and deprecated options, which can lead to confusion.
The recommended modern way to describe disks is to use a combination of
The most explicit way to describe disks is to use a combination of
``-device`` to specify the hardware device and ``-blockdev`` to
describe the backend. The device defines what the guest sees and the
backend describes how QEMU handles the data.
backend describes how QEMU handles the data. It is the only guaranteed
stable interface for describing block devices and as such is
recommended for management tools and scripting.
The ``-drive`` option combines the device and backend into a single
command line option which is a more human friendly. There is however no
interface stability guarantee although some older board models still
need updating to work with the modern blockdev forms.
Older options like ``-hda`` are essentially macros which expand into
``-drive`` options for various drive interfaces. The original forms
bake in a lot of assumptions from the days when QEMU was emulating a
legacy PC, they are not recommended for modern configurations.
ERST
@ -1639,6 +1651,14 @@ SRST
the raw disk image you use is not written back. You can however
force the write back by pressing C-a s (see the :ref:`disk images`
chapter in the System Emulation Users Guide).
.. warning::
snapshot is incompatible with ``-blockdev`` (instead use qemu-img
to manually create snapshot images to attach to your blockdev).
If you have mixed ``-blockdev`` and ``-drive`` declarations you
can use the 'snapshot' property on your drive declarations
instead of this global option.
ERST
DEF("fsdev", HAS_ARG, QEMU_OPTION_fsdev,