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Re: [Qemu-block] [Qemu-devel] [PATCH 2/2] Add file-posix-dynamic-auto-re

From: Markus Armbruster
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 2/2] Add file-posix-dynamic-auto-read-only feature
Date: Fri, 29 Mar 2019 14:14:36 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Markus Armbruster <address@hidden> writes:

> Kevin Wolf <address@hidden> writes:
>> auto-read-only=on changed its behaviour in file-posix for the 4.0
>> release.
> Commit hash, please.

I guess it's commit 23dece19da4 "file-posix: Make auto-read-only

>>          This change cannot be detected through the usual mechanisms
>> like schema introspection. Add a new feature to query-qemu-features to
>> allow libvirt to detect the presence of the new behaviour.
>> Signed-off-by: Kevin Wolf <address@hidden>
>> ---
>>  qapi/misc.json | 7 ++++++-
>>  qmp.c          | 1 +
>>  2 files changed, 7 insertions(+), 1 deletion(-)
>> diff --git a/qapi/misc.json b/qapi/misc.json
>> index d892f37633..df23c54a65 100644
>> --- a/qapi/misc.json
>> +++ b/qapi/misc.json
>> @@ -3058,10 +3058,15 @@
>>  # Information about support for QEMU features that isn't available through
>>  # schema introspection.
>>  #
>> +# @file-posix-dynamic-auto-read-only:
>> +#   true if auto-read-only=on means that the image file is dynamically 
>> reopened
>> +#   read-only or read-write depending on whether any writers are attached to
>> +#   the node.
>> +#
> The name @file-posix-dynamic-auto-read-only suggests the semantic change
> of auto-read-only=on explained in the doc comment applies only to some
> BlockDrivers (as a non-variant member of BlockdevOptions, auto-read-only
> applies to all).  Which ones exactly?

Kevin answered this on IRC.  In my words (and possibly with my

@auto-read-only is advisory, as documented:

# @auto-read-only: if true and @read-only is false, QEMU may automatically
#                  decide not to open the image read-write as requested, but
#                  fall back to read-only instead (and switch between the modes
#                  later), e.g. depending on whether the image file is writable
#                  or whether a writing user is attached to the node
#                  (default: false, since 3.1)

Note the "QEMU may".

The file-posix drivers always honor it.  These are "file",
"host_device", "host_cdrom" on a POSIX host, but not on a Windows host.
No other driver does.

The semantic change we're trying to convey is the following.  Before
commit 23dece19da4, these drivers did not make use of "(and switch
between the modes later)".  Since then, they do: they switch from
read-only to read/write only when the first writer appears, and switch
back to read-only when the last writer disappears.

The "posix" in @file-posix-dynamic-auto-read-only feels weird.  To make
sense of it, you need to know on what kind of host QEMU runs.  If we
ever implement it for Windows hosts, we'd get do add

What about instead adding @file-dynamic-auto-read-only with a suitable
compile-time conditional?


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