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Re: [Qemu-devel] [PATCH RFC v3 for-2.9 08/11] rbd: Revert -blockdev and

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH RFC v3 for-2.9 08/11] rbd: Revert -blockdev and -drive parameter auth-supported
Date: Mon, 27 Mar 2017 14:27:28 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

On 03/27/2017 02:14 PM, Markus Armbruster wrote:

>>>> Removing the .json QMP support is fine. But I'm reluctant to give R-b
>>>> without knowing for sure that -drive usage won't regress.
>>> auth-supported landed in master only on March 2nd.
>> auth-supported via -blockdev-add only landed on March 2nd.  But
>> auth-supported via -drive landed in commit 60390a2, Jan 2016; and is in
>> use by libvirt:
>> src/qemu/qemu_command.c:
>> ":key=%s:auth_supported=cephx\\;none",
> That's a key-value part of the pseudo-filename.  *Not* reverted by this
> patch.  Only QemuOpts parameter auth_supported is.
> QemuOpts parameter: -drive driver=rbd,auth_supported.0.auth=none,...
> Pseudo-filename:    -drive file=rbd:...:auth_supported=none
>>> What libvirt usage exactly do you think this could break?
>> Libvirt has been managing rbd drives using -drive since at least libvirt
>> commit 5745cd1, in Nov 2011, where even back then it was passing:
>>             virBufferEscape(opt, ":", ":key=%s:auth_supported=cephx none",
>> (back when it used space instead of ; to separate the list of supported
>> auth types).

Ah, so as long as libvirt uses 'drive file=rbd:...' with key-value
pairs, our backdoor of =key-values will let it continue to work.  And
looking more at libvirt, it definitely looks like it is sticking to
file=rbd: pseudo-filenames for now; for example:


I'm not sure if (or where) a big fat comment to that effect would be
beneficial, but it certainly goes a long ways in explaining the goals of
your series.

>> As I've never personally used RBD (whether through qemu directly, or
>> through libvirt), I'm extremely wary of breaking -drive usage that
>> "works" (for some definition of "works"), even though I have no qualms
>> making the QMP interface extremely limited.
> All clear now?

I think so.  Commit 60390a2 (Jan 2016) mentions the use of
auth_supported in the commit message (matching libvirt usage), but under
the pseudo-file format; the actual rbd.c file did not special case it
differently from any other key-value pair.  "auth" wasn't added to
QemuOpts (for the -drive driver=rbd, form) until commit 0a55679, which
is unreleased, so libvirt can't have been relying on it, and ripping it
out now is safe enough.

Under that light, I'm now going to re-read your patch...

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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