[libvirt] QEMU capabilities vs machine types

[Daniel P. Berrange]

On Wed, Feb 11, 2015 at 05:09:01PM +0100, Michal Privoznik wrote:
> On 11.02.2015 16:47, Daniel P. Berrange wrote:
> > On Wed, Feb 11, 2015 at 04:31:53PM +0100, Michal Privoznik wrote:
> >>
> > 
> > There are two reasons why we query & check the supported capabilities
> > from QEMU
> > 
> >  1. There are multiple possible CLI args for the same feature and
> >     we need to choose the "best" one to use
> > 
> >  2. The feature is not supported and we want to give the caller a
> >     better error message than they'd get from QEMU
> > 
> > I'm unclear from the bug which scenario applies here.
> > 
> > If it is scenario 2 though, I'd just mark it as CANTFIX or WONTFIX,
> > as no matter what we do the user would get an error. It is not worth
> > making our capability matrix a factor of 10+ bigger just to get a
> > better error message.
> > 
> > If it is scenario 1, I think the burden is on QEMU to solve. The
> > memory-backend-{file,ram} CLI flags shouldn't be tied to guest
> > machine types, as they are backend config setup options that should
> > not impact guest ABI.
> 
> It's somewhere in between 1 and 2. Back in RHEL-7.0 days libvirt would
> have created a guest with:
> 
> -numa node,...,mem=1337
> 
> But if qemu reports it support memory-backend-ram, libvirt tries to use it:
> 
> -object memory-backend-ram,id=ram-node0,size=1337M,... \
> -numa node,...,memdev=ram-node0
> 
> This breaks migration to newer qemu which is in RHEL-7.1. If qemu would
> report the correct value, we can generate the correct command line and
> migration succeeds. However, our fault is, we are not asking the correct
> question anyway.

Ah so the problem is rather than QEMU's migration data stream was not
compatible for these two possible NUMA setups, despite it not affecting
guest ABI.  In general I'd like to think we'd not get into this situation
in the first place. We'd probably have to just special case the code based
on machine type in this case though


Regards,
Daniel
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