Re: [Qemu-devel] [PATCH V4 01/10] NUMA: Support multiple CPU ranges on -numa option

[Paolo Bonzini]

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Il 15/07/2013 23:33, Eric Blake ha scritto:
>>>>> Newer libvirt can be taught to append 'M' when it detects
>>>>> it is talking to newer qemu.  While you have a point that
>>>>> it is annoying to force users to upgrade to a newer libvirt
>>>>> merely because they upgraded qemu, the libvirt point of
>>>>> view is that the following are supported:
>>>>> 
>>>>> old libvirt -> old qemu new libvirt -> old qemu new libvirt
>>>>> -> new qemu
>>>>> 
>>>>> but that this combination is always best effort and not
>>>>> required to work:
>>>>> 
>>>>> old libvirt -> new qemu
>>> 
>>> I don't think this is the case, unless you're talking of *very*
>>> old libvirt (e.g. pre-QMP).
> As a counter-example, I can recall a case where a qemu release that
> used just two digits (was that 1.2?) broke operation under older
> libvirt that assumed versions would always be three digits; but it
> definitely occurred after 0.15.x which is the point at which
> libvirt started favoring QMP.  That is, we had a case in Fedora
> where if you upgraded qemu, you HAD to also update libvirt to be
> able to keep your guests running.

Right, I remember that now.  So, better: "we have some interfaces
which are considered API, and old libvirt -> new QEMU should not break
for things that use those interfaces".  QMP and the command line are
definitely one.

The case you mentioned was about -help, if I remember correctly, which
was indeed quite brittle (like HMP).

> But yes, the goal of having command line compatibility, so that
> any application using the same command line it always uses will get
> the same guest, regardless of a qemu upgrade in the meantime,
> should be our default mode of operation, even if newer apps should
> prefer newer (better) command line interfaces.

Yes, the command line *is* part of the API.

Paolo
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