Re: [PATCH 1/2] KVM: Expose MCE control MSRs to userspace

[Huang Ying <ying.huang@intel.com>]

On Thu, 2010-07-08 at 16:16 +0800, Avi Kivity wrote:
> On 07/08/2010 11:03 AM, Huang Ying wrote:
> > On Thu, 2010-07-08 at 15:43 +0800, Avi Kivity wrote:
> >    
> >> On 07/08/2010 05:07 AM, Huang Ying wrote:
> >>      
> >>>        
> >>>>    static u32 emulated_msrs[] = {
> >>>>    	MSR_IA32_MISC_ENABLE,
> >>>> +	MSR_IA32_MCG_STATUS,
> >>>> +	MSR_IA32_MCG_CTL,
> >>>>
> >>>>          
> >>> We need only clear MSR_IA32_MCG_STATUS during reset, but should not
> >>> clear MSR_IA32_MCG_CTL.
> >>>
> >>>
> >>>        
> >> Why not?
> >>      
> > According to Intel 64 and IA32 Architectures Software Developer's Manual
> > (SDM) Vol 3A (Table 9-1), machine check MSRs should be sticky across
> > reset.
> 
> 
> Well, my copy says:
> 
>      Machine-Check Architecture   Pwr up or Reset:  Undefined          
> INIT: Unchanged

If my understanding is correct. Pwr up or Reset is cold reboot and INIT
is warm reboot. So MCE can be logged after the warm reboot.

> So it seems we're free to clear it.  But probably the best thing would 
> be to have the bios initialize it to disabled state?
> 
> Note, in any case we have to expose the MSRs for live migration.
> 
> > Except we need some special processing for MSR_IA32_MCG_STATUS.
> >    
> 
> What do you have in mind?

MSR_IA32_MCG_STATUS need to be cleared across reboot.

> > And if we clear MSR_IA32_MCG_CTL, the machine check reporting is
> > disabled according to SDM Vol 3A, section 15.3.1.3
> >    
> 
> Won't the kernel reenable MCE?  In my testing, the sequence 
> MCE-reset-MCE worked after the patch (whereas it would fail without it).

Yes. Because kernel will reenable it. But if we only clear
MSR_IA32_MCG_STATUS only, MCE-reset-MCE should work too.

Best Regards,
Huang Ying