Re: [PATCH v2 3/3] KVM: VMX: enable guest access to LMCE related MSRs

[Haozhong Zhang]

On 06/16/16 11:55, Eduardo Habkost wrote:
> On Thu, Jun 16, 2016 at 12:04:50PM +0200, Paolo Bonzini wrote:
> > On 16/06/2016 08:05, Haozhong Zhang wrote:
> > > From: Ashok Raj <ashok.raj@xxxxxxxxx>
> > > 
> > > On Intel platforms, this patch adds LMCE to KVM MCE supported
> > > capabilities and handles guest access to LMCE related MSRs.
> > > 
> > > Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> > > [Haozhong: macro KVM_MCE_CAP_SUPPORTED => variable kvm_mce_cap_supported
> > >            Only enable LMCE on Intel platform
> > > 	   Check MSR_IA32_FEATURE_CONTROL when handling guest
> > > 	     access to MSR_IA32_MCG_EXT_CTL]
> > > Signed-off-by: Haozhong Zhang <haozhong.zhang@xxxxxxxxx>
> [...]
> > > @@ -6433,6 +6455,8 @@ static __init int hardware_setup(void)
> > >  
> > >  	kvm_set_posted_intr_wakeup_handler(wakeup_handler);
> > >  
> > > +	kvm_mce_cap_supported |= MCG_LMCE_P;
> > 
> > Ah, so virtual LMCE is available on all processors!  This is
> > interesting, but it also makes it more complicated to handle in QEMU; a
> > new QEMU generally doesn't require a new kernel.
> > 
> > Eduardo, any ideas?
> 
> (CCing libvirt list)
> 
> As we shouldn't make machine-type changes introduce new host
> requirements, it looks like we need to either add a new set of
> CPU models (unreasonable), or expect management software to
> explicitly enable LMCE after ensuring the host supports it.
>
> Or we could wait for a reasonable time after the feature is
> available in the kernel, and declare that QEMU as a whole
> requires a newer kernel. But how much time would be reasonable
> for that?
> 
> Long term, I believe we should think of a better solution. I
> don't think it is reasonable to require new libvirt code to be
> written for every single low-level feature that requires a newer
> kernel or newer host hardware. Maybe new introspection interfaces
> that would allow us to drop the "no new requirements on
> machine-type changes" rule?
>

Because new MSR (MSR_IA32_MCG_EXT_CTL) and new MSR bit
(FEATURE_CONTROL_LMCE in MSR_IA32_FEATURE_CONTROL) are introduced by
LMCE, QEMU requires new KVM which can handle those changes.

I'm not familiar with libvirt. Does the requirement of new KVM
capability bring any troubles to libvirt?

Thanks,
Haozhong