Re: [PATCH] Add MCE support to KVM

[Huang Ying]

On Mon, 2009-04-13 at 21:02 +0800, Avi Kivity wrote:
> Huang Ying wrote:
> > On Sat, 2009-04-11 at 20:04 +0800, Avi Kivity wrote:
> >   
> >>>> Why not always allocate it on vcpu setup?
> >>>>     
> >>>>         
> >>> Because the MCE bank number is not fixed, it is based on mcg_cap from
> >>> user space.
> >>>   
> >>>       
> >> Right, but we can allocate the maximum number, no?  it's a fairly small 
> >> amount of memory.
> >>     
> >
> > OK. If you are OK with 8k extra memory usage.
> >   
> 
> Let's make it 64 banks like Andi suggests.  Return the number in the 
> KVM_CHECK_EXTENSION call (we do the same for memory slots) so userspace 
> can adjust.

OK. I will change this.

> >>
> >> kvm does feature negotiation (really, feature advertising) using 
> >> KVM_CAP_... and KVM_CHECK_EXTENSION.  So there's no need for this.
> >>     
> >
> > It is not only feature advertising, but also something like CPUID. Based
> > on CPU type specified in command line (qemu -cpu ?), different mcg_cap
> > can be used to determine number of MCE banks and various features of MCE
> > (MCG_CTL_P, MCG_TES_P, MCG_CMCI, etc) for specified CPU type. With KVM
> > feature advertising, we can only check whether MCE simulation is
> > supported, but can not set parameters of MCE simulation.
> >
> > Maybe this interface can be changed to KVM_SET_MSRS, with
> > MSR_IA32_MCG_CAP be set by kvm-qemu. But MSR_IA32_MCG_CAP is read-only
> > by spec, and current interface can not distinguishes between MSR set
> > from guest system and MSR setup from kvm-qemu. I think there should be
> > other read-only MSRs need to be setup during guest system
> > creation/setup. How about add a interface such as KVM_SETUP_MSRS to
> > setup the value of read-only MSRs?
> >   
> 
> Then we would need to tell which read-only MSRs are setup writeable and 
> which aren't...

Maybe something like only if KVM_CAP_MCE is advertised, the MCE related
read-only RO-MSRs can be setup. Other RO-MSRs setup code can be added
following the similar rules.

And interface is something like follow?

long kvm_arch_vcpu_ioctl(...)
{
    ...
    case KVM_SETUP_RO_MSRS:
        r = msr_io(vcpu, argp, kvm_setup_ro_msr, 0);
    ...
}

int kvm_setup_ro_msr(...)
{
    /* return 1 for setup un-writable MSR, otherwise 0 */
}

> I'm okay with an ioctl to setup MCE, but just make sure userspace has 
> all the information to know what the kernel can do rather than the 
> try-and-see-if-it-works approach.  We can publish this information via 
> KVM_CAP things, or via another ioctl (see KVM_GET_SUPPORTED_CPUID2 for 
> an example).

Yes. MCE support should be published by KVM_CAP_MCE and other features
can be published via reading the default value of MSR_IA32_MCG_CAP.

Best Regards,
Huang Ying

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