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

From: Peter Krempa
Subject: Re: [Qemu-block] [PATCH v2 10/10] file-posix: Make auto-read-only dynamic
Date: Mon, 11 Mar 2019 21:25:34 +0100
User-agent: Mutt/1.11.3 (2019-02-01)

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.

> 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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