Re: [libvirt] [PATCH 6/8] virQEMUCapsSetFromNodes: Skip setting deprecated flags
On 3/7/19 1:07 PM, Daniel P. Berrangé wrote:
On Thu, Mar 07, 2019 at 12:52:46PM +0100, Erik Skultety wrote:
>> However, looking at the bigger picture is this a safe thing to do? I mean,
>> imagine the following scenario:
>>
>> 1) say there is capability X that affects certain part of cmd line. And
>> assume that those two possibilities are incompatible. If cmd line is
>> generated one way then migration to a qemu which has cmd line generated the
>> other way fails.
>>
>> 2) in release R we deprecate X and thus do not format it in
<capabilities/>
>> in status XML.
>>
>> 3) user starts a domain D.
>>
>> 4) user saves D into a file
>>
>> 5) sysadmin downgrades libvirt to R-1
>
> Do we even support downgrade this way? I know we migrate to older version but
> isn't that different?
Okay, forget about save+restore. Let's just keep the domain D running
and then downgrade libvirt. Now, instead of X affecting cmd line let it
affect how we talk to qemu on monitor. Since R assumed X always being
there then X wouldn't be recorded in status XML. Later when R-1 daemon
is starting and parsing the status XML it won't find X and thus might
make wrong decissions. For instance, if X would be some capability that
affects the way we generate PCI addresses for guest devices, then this
would really harm the user.
Worse, let just start the domain with R-1, then upgrade to R to test it,
call an API over D (at which point the status XML is regenerated, now
without X stored in it), find that it's not working so roll back to R-1
and puff. The capability is gone.
Yeah, downgrades of libvirt are not something we claim is supported. If
will often work but we're not guaranteeing it & have broken it in the
past, especially for running guests. You might be lucky if you have
upgraded & immedaitely downgrade, but if you've made changes to guests
while the new libvirt was running all bets are off.
Not even in the case I'm describing above?
Michal