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Re: [Qemu-block] [Qemu-devel] [PATCH v2 10/10] file-posix: Make auto-rea

From: Markus Armbruster
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v2 10/10] file-posix: Make auto-read-only dynamic
Date: Mon, 11 Mar 2019 22:15:32 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Peter Krempa <address@hidden> writes:

> On Mon, Mar 11, 2019 at 15:10:36 -0500, Eric Blake wrote:
>> 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
>> compilation.
>> https://lists.gnu.org/archive/html/qemu-devel/2019-03/msg02510.html
> Oh, bummer, I missed that it was no longer needed. I still think it's
> worth adding that for future use once it will be necessary to detect
> that certain things were patched and require libvirt to change behaviour
> if that's the case.

We'll add it when we have a compelling use for it.

>> 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.

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