Re: [Qemu-devel] [libvirt] inconsistent handling of "qemu64" CPU model

From: Kashyap Chamarthy <kchamart@redhat.com>
To: Chris Friesen <chris.friesen@windriver.com>
Cc: libvir-list@redhat.com, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [libvirt] inconsistent handling of "qemu64" CPU model
Date: Thu, 26 May 2016 11:45:46 +0200	[thread overview]
Message-ID: <20160526094546.slj7r3rkpv24ihjx@eukaryote> (raw)
In-Reply-To: <574685F4.8070304@windriver.com>

On Wed, May 25, 2016 at 11:13:24PM -0600, Chris Friesen wrote:

[...]

> However, if I explicitly specify a custom CPU model of "qemu64" the
> instance refuses to boot and I get a log saying:

[Not a direct answer to the exact issue you're facing, but a related
issue that is being investigated presently...]

Currently there's a related (regression) in upstream libvirt 1.3.4:

The crux of the issue here is: the libvirt custom 'gate64' model is not
being translated into a CPU definition that QEMU can recognize
(which you can find from `qemu-system-x86 -cpu \?`).

See this bug (it has reproducer, and discussion):

    https://bugzilla.redhat.com/show_bug.cgi?id=1339680 -- libvirt CPU
    driver fails to translate a custom CPU model into something that
    QEMU recognizes

The bug (regression) is bisected, by Jiri Denemark, to this commit:

    v1.2.9-31-g445a09b "qemu: Don't compare CPU against host for TCG".

> libvirtError: unsupported configuration: guest and host CPU are not
> compatible: Host CPU does not provide required features: svmlibvirtError:
> unsupported configuration: guest and host CPU are not compatible: Host CPU
> does not provide required features: svm

> 
> When this happens, some of the XML for the domain looks like this:
>   <os>
>     <type arch='x86_64' machine='pc-i440fx-utopic'>hvm</type>
>  ....
> 
>   <cpu mode='custom' match='exact'>
>     <model fallback='allow'>qemu64</model>
>     <topology sockets='1' cores='1' threads='1'/>
>   </cpu>
> 
> Of course "svm" is an AMD flag and I'm running an Intel CPU.  But why does
> it work when I just rely on the default virtual CPU?  Is
> kvm_default_unset_features handled differently when it's implicit vs
> explicit?
> 
> If I explicitly specify a custom CPU model of "kvm64" then it boots, but of
> course I get a different virtual CPU from what I get if I don't specify
> anything.
> 
> Following some old suggestions I tried turning off nested kvm, deleting
> /var/cache/libvirt/qemu/capabilities/*, and restarting libvirtd.  Didn't
> help.
> 
> So...anyone got any ideas what's going on?  Is there no way to explicitly
> specify the model that you get by default?
> 
> 
> Thanks,
> Chris
> 
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-- 
/kashyap