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

On Tue, Dec 16, 2014 at 02:47:24PM +0000, Daniel P. Berrange wrote: > 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. But this means that VMs that have e.g. <cpu mode='custom' match='exact'> <model fallback='forbid'>Haswell</model> <vendor>Intel</vendor> ... </cpu> refuse to start after the microcode upgrade like error: Failed to start domain foo error: unsupported configuration: guest and host CPU are not compatible: Host CPU does not provide required features: hle Following your argument this makes a lot of sense since the feature isn't in the set of supported features anymore.

From a users perspective it's confusing since they have a Haswell CPU built in still. So should we define a Haswell2 cpu type without hle so users can select it in case they don't want to use 'host-model'?