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Re: [Qemu-block] [PATCH v2 10/10] file-posix: Make auto-read-only dynami

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH v2 10/10] file-posix: Make auto-read-only dynamic
Date: Mon, 11 Mar 2019 15:10:36 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1

On 3/11/19 2:59 PM, Peter Krempa wrote:

>> auto-read-only was introduced in 3.1, at which point we intentionally
>> had sufficiently loose wording to permit (but not require) dynamic state
>> checking; so you are not breaking the interface.  On the other hand, is
>> libvirt going to have problems introspecting whether it can use
>> auto-read-only and get the dynamic behavior it needs?  Or is there
>> enough else in the way of libvirt's switch to -blockdev that it won't
>> attempt anything that needs auto-read-only without other 4.0 interfaces
>> anyway, at which point detecting the presence of the field (but not
>> whether the field has a guarantee of dynamic behavior) on 3.1 doesn't
>> matter?
> I think we can use Stefan's capability detection mechanism he introduced
> for the migration with cache enabled for local files to add a flag for
> this as well.

Except I thought we decided that the most recent version of his QMP
changes was now fully-introspectible, thanks to using conditional


Well, that may prove to be a short-lived hiatus, if libvirt would
happily attempt to use qemu 3.1 and fail without some other
introspectible hook to know whether auto-read-only has required semantics.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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