Re: [PATCH] KVM: x86: Add support for CMCI and UCNA.

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 12, 2022, Paolo Bonzini wrote:
> On 4/11/22 21:08, Sean Christopherson wrote:
> > > +			if (!(mcg_cap & MCG_CMCI_P) &&
> > > +			    (data || !msr_info->host_initiated))
> > This looks wrong, userspace should either be able to write the MSR or not, '0'
> > isn't special.  Unless there's a danger to KVM, which I don't think there is,
> > userspace should be allowed to ignore architectural restrictions, i.e. bypass
> > the MCG_CMCI_P check, so that KVM doesn't create an unnecessary dependency between
> > ioctls.  I.e. this should be:
> > 
> > 		if (!(mcg_cap & MCG_CMCI_P) && !msr_info->host_initiated)
> > 			return 1;
> > 
> 
> This is somewhat dangerous as it complicates (or removes) the invariants
> that other code can rely on.  Thus, usually, only the default value is
> allowed for KVM_SET_MSR.

Heh, I don't know if "usually" is the right word, that implies KVM is consistent
enough to have a simple majority for any behavior, whatever that behavior may be :-)

Anyways, on second look, I agree that KVM should require that userspace first enable
CMCI via mcg_cap.  I thought that vcpu->arch.mcg_cap could be written via the MSR
interface, i.e. via userspace writes to MSR_IA32_MCG_CAP, and could create dependencies
within KVM_SET_MSRS.  But KVM only allows reading the MSR.