[libvirt] [PATCH] Drop hle feature for Haswell CPUs

[Daniel P. Berrange]

On Tue, Dec 16, 2014 at 03:37:06PM +0100, Guido Günther wrote:
> On Tue, Dec 16, 2014 at 12:40:26PM +0000, Daniel P. Berrange wrote:
> > On Tue, Dec 16, 2014 at 01:33:15PM +0100, Guido Günther wrote:
> > > The intel-microcode 3.20140913.1 update disables TSX-NI (transactional
> > > memory instructions). When a server running libvirt is rebooted with
> > > this update, libvirt no longer considers the machine to have a Haswell
> > > CPU:
> > > 
> > >  # virsh capabilities | grep -A1 '<arch>x86_64'
> > >     <arch>x86_64</arch>
> > >     <model>SandyBridge</model>
> > > 
> > > Since Intel disables the feature on their CPUs we shouldn't check for it
> > > as well to keep VMs using Haswell working.
> > > 
> > > This was debugged and reported by Chris Boot at
> > > http://bugs.debian.org/773189
> > > ---
> > >  src/cpu/cpu_map.xml | 1 -
> > >  1 file changed, 1 deletion(-)
> > > 
> > > diff --git a/src/cpu/cpu_map.xml b/src/cpu/cpu_map.xml
> > > index bd9b056..f41dbce 100644
> > > --- a/src/cpu/cpu_map.xml
> > > +++ b/src/cpu/cpu_map.xml
> > > @@ -507,7 +507,6 @@
> > >        <feature name='movbe'/>
> > >        <feature name='fsgsbase'/>
> > >        <feature name='bmi1'/>
> > > -      <feature name='hle'/>
> > >        <feature name='avx2'/>
> > >        <feature name='smep'/>
> > >        <feature name='bmi2'/>
> > 
> > NACK.  We can not change existing models in cpu_map.xml because that
> > results in a guest ABI change.
> 
> But Intel changed the ABI as well by removing hle so what would we do
> to move forward? Introduce another model? This would still break
> systems with the microcode update.

Simply do nothing. There is no requirement that guest CPU model names
match the host CPU you are running on. A guest CPU model name is simply
a synonym for a collection of features. If libvirt reports 'SandyBridge'
for the host CPU model, when it is in fact 'Hasmere' that's not a serious
problem, because the key fact is that the feature bits are correctly
detected regardless of what name is reported.

Regards,
Daniel
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