Re: [PATCH 3/7 v7] KVM: nSVM: No need to set bits 11:0 in MSRPM and IOPM bitmaps

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 22, 2021, Krish Sadhukhan wrote:
> On 4/22/21 11:01 AM, Sean Christopherson wrote:
> > 		offset = svm->nested.ctl.msrpm_base_pa + (p * 4);
> > 
> > 		if (kvm_vcpu_read_guest(&svm->vcpu, offset, &value, 4)) <- This reads vmcb12
> > 			return false;
> > 
> > 		svm->nested.msrpm[p] = svm->msrpm[p] | value; <- Merge vmcb12's bitmap to KVM's bitmap for L2

... 
 
> Getting back to your concern that this patch breaks
> nested_svm_vmrun_msrpm().  If L1 passes a valid address in which some bits
> in 11:0 are set, the hardware is anyway going to ignore those bits,
> irrespective of whether we clear them (before my patch) or pass them as is
> (my patch) and therefore what L1 thinks as a valid address will effectively
> be an invalid address to the hardware. The only difference my patch makes is
> it enables tests to verify hardware behavior. Am missing something ?

See the above snippet where KVM reads the effectively vmcb12->msrpm to merge L1's
desires with KVM's desires.  By removing the code that ensures
svm->nested.ctl.msrpm_base_pa is page aligned, the above offset calculation will
be wrong.