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 10:50 AM, Krish Sadhukhan wrote:
> > 
> > On 4/20/21 1:00 PM, Sean Christopherson wrote:
> > > On Mon, Apr 12, 2021, Krish Sadhukhan wrote:
> > > > According to APM vol 2, hardware ignores the low 12 bits in
> > > > MSRPM and IOPM
> > > > bitmaps. Therefore setting/unssetting these bits has no effect
> > > > as far as
> > > > VMRUN is concerned. Also, setting/unsetting these bits prevents
> > > > tests from
> > > > verifying hardware behavior.
> > > > 
> > > > Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
> > > > ---
> > > >   arch/x86/kvm/svm/nested.c | 2 --
> > > >   1 file changed, 2 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > > > index ae53ae46ebca..fd42c8b7f99a 100644
> > > > --- a/arch/x86/kvm/svm/nested.c
> > > > +++ b/arch/x86/kvm/svm/nested.c
> > > > @@ -287,8 +287,6 @@ static void
> > > > nested_load_control_from_vmcb12(struct vcpu_svm *svm,
> > > >         /* Copy it here because nested_svm_check_controls will
> > > > check it.  */
> > > >       svm->nested.ctl.asid           = control->asid;
> > > > -    svm->nested.ctl.msrpm_base_pa &= ~0x0fffULL;
> > > > -    svm->nested.ctl.iopm_base_pa  &= ~0x0fffULL;
> > > This will break nested_svm_vmrun_msrpm() if L1 passes an unaligned
> > > address.
> > > The shortlog is also wrong, KVM isn't setting bits, it's clearing bits.
> > > 
> > > I also don't think svm->nested.ctl.msrpm_base_pa makes its way to
> > > hardware; IIUC,
> > > it's a copy of vmcs12->control.msrpm_base_pa.  The bitmap that gets
> > > loaded into
> > > the "real" VMCB is vmcb02->control.msrpm_base_pa.
> > 
> > 
> > Not sure if there's a problem with my patch as such, but upon inspecting
> > the code, I see something missing:
> > 
> >     In nested_load_control_from_vmcb12(), we are not really loading
> > msrpm_base_pa from vmcb12 even     though the name of the function
> > suggests so.
> > 
> >     Then nested_vmcb_check_controls() checks msrpm_base_pa from
> > 'nested.ctl' which doesn't have         the copy from vmcb12.
> > 
> >     Then nested_vmcb02_prepare_control() prepares the vmcb02 copy of
> > msrpm_base_pa from vmcb01.ptr->control.msrpm_base_pa.
> > 
> >     Then nested_svm_vmrun_msrpm() uses msrpm_base_pa from 'nested.ctl'.
> > 
> > 
> > Aren't we actually using msrpm_base_pa from vmcb01 instead of vmcb02 ?
> 
> 
> Sorry, I meant to say,  "from vmcb01 instead of vmcb12"

The bitmap that's shoved into hardware comes from vmcb02, the bitmap that KVM
reads to merge into _that_ bitmap comes from vmcb12.

static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
{
	/*
	 * This function merges the msr permission bitmaps of kvm and the
	 * nested vmcb. It is optimized in that it only merges the parts where
	 * the kvm msr permission bitmap may contain zero bits
	 */
	int i;

	if (!(vmcb_is_intercept(&svm->nested.ctl, INTERCEPT_MSR_PROT)))
		return true;

	for (i = 0; i < MSRPM_OFFSETS; i++) {
		u32 value, p;
		u64 offset;

		if (msrpm_offsets[i] == 0xffffffff)
			break;

		p      = msrpm_offsets[i];
		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
	}

	svm->vmcb->control.msrpm_base_pa = __sme_set(__pa(svm->nested.msrpm)); <- This is vmcb02

	return true;
}