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

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 06/17/16 14:15, Eduardo Habkost wrote:
> On Fri, Jun 17, 2016 at 09:11:16AM +0800, Haozhong Zhang wrote:
> > 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@intel.com>
> > > > > 
> > > > > 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@intel.com>
> > > > > [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@intel.com>
> > > [...]
> > > > > @@ -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.
> 
> If I understood correctly, you are describing the second option
> above (declaring that QEMU as a whole requires a newer kernel).
> 
> > 
> > I'm not familiar with libvirt. Does the requirement of new KVM
> > capability bring any troubles to libvirt?
>
> It does, assuming that we still support running QEMU under an
> older kernel where KVM doesn't LMCE. In this case, the pc-2.6
> machine-type will run, but the pc-2.7 machine-type won't.
>
> The requirement of new KVM capabilities based on the machine-type
> is a problem for livirt. libvirt have some host-capabilities APIs
> to allow software to check if the VM can be run on (or migrated
> to) a host, but none of them are based on machine-type.
> 
> This is not necessarily specific to libvirt: people may have
> their own configuration or scripts that use the default "pc"
> alias, and a QEMU upgrade shouldn't break their configuration.
>

Thanks for the explanation!

If we disable LMCE in QEMU by default (even for -cpu host), will it
still be a problem? That is,

- pc-2.7 can continue to run on old kernels unless users explicitly
  require LMCE

- existing libvirt VM configurations can continue to work on pc-2.7
  because LMCE is not specified in those configurations and are
  disabled by default (i.e. no requirement for new kernels)

- existing QEMU configurations/scripts using pc alias can continue to
  work on pc-27 for the same reason above.

Thanks,
Haozhong