Re: [PATCH v4 09/20] KVM:x86: Add common code of CET MSR access

[Yang, Weijiang]

On 7/26/2023 9:46 PM, Chao Gao wrote:
> On Wed, Jul 26, 2023 at 04:26:06PM +0800, Yang, Weijiang wrote:
> > > > +	/*
> > > > +	 * This function cannot work without later CET MSR read/write
> > > > +	 * emulation patch.
> > > Probably you should consider merging the "later" patch into this one.
> > > Then you can get rid of this comment and make this patch easier for
> > > review ...
> > Which later patch you mean? If you mean [13/20] KVM:VMX: Emulate read and
> > write to CET MSRs,
> >
> > then I intentionally separate these two, this one is for CET MSR common
> > checks and operations,
> >
> > the latter is specific to VMX, and add the above comments in case someone is
> The problem of this organization is the handling of S_CET, SSP, INT_SSP_TABLE
> MSR is incomplete in this patch. I think a better organization is to either
> merge this patch and patch 13, or move all changes related to S_CET, SSP,
> INT_SSP_TABLE into patch 13? e.g.,

Yes, I'm thinking of merging this patch with patch 13 to make it 
complete, thanks for

the suggestion!

> 	case MSR_IA32_U_CET:
> -	case MSR_IA32_S_CET:
> 		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
> 			return 1;
> 		if ((!guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
> 		     (data & CET_SHSTK_MASK_BITS)) ||
> 		    (!guest_can_use(vcpu, X86_FEATURE_IBT) &&
> 		     (data & CET_IBT_MASK_BITS)))
> 			return 1;
> -		if (msr == MSR_IA32_U_CET)
> -			kvm_set_xsave_msr(msr_info);
> 		kvm_set_xsave_msr(msr_info);
> 		break;
> -	case MSR_KVM_GUEST_SSP:
> -	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> 		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
> 			return 1;
> 		if (is_noncanonical_address(data, vcpu))
> 			return 1;
> 		if (!IS_ALIGNED(data, 4))
> 			return 1;
> 		if (msr == MSR_IA32_PL0_SSP || msr == MSR_IA32_PL1_SSP ||
> 		    msr == MSR_IA32_PL2_SSP) {
> 			vcpu->arch.cet_s_ssp[msr - MSR_IA32_PL0_SSP] = data;
> 		} else if (msr == MSR_IA32_PL3_SSP) {
> 			kvm_set_xsave_msr(msr_info);
> 		}
> 		break;
>
>
>
> BTW, shouldn't bit2:0 of MSR_KVM_GUEST_SSP be 0? i.e., for MSR_KVM_GUEST_SSP,
> the alignment check should be IS_ALIGNED(data, 8).

The check for GUEST_SSP should be consistent with that of PLx_SSPs, 
otherwise there would

be issues, maybe I need to change the alignment check as :

#ifdef CONFIG_X86_64

if (!IS_ALIGNED(data, 8))
	return 1;
#else
if (!IS_ALIGNED(data, 4))

	return 1;
#endif

> > bisecting
> >
> > the patches and happens to split at this patch, then it would faulted and
> > take some actions.
> I am not sure what kind of issue you are worrying about. In my understanding,
> KVM hasn't advertised the support of IBT and SHSTK, so,
> kvm_cpu_cap_has(X86_FEATURE_SHSTK/IBT) will always return false. and then
> kvm_cet_is_msr_accessible() is guaranteed to return false.
>
> If there is any issue in your mind, you can fix it or reorganize your patches to
> avoid the issue. To me, adding a comment and a warning is not a good solution.

I will reorganize the patches and merge the code in this patch to patch 13.

> > > > int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > > > {
> > > > 	u32 msr = msr_info->index;
> > > > @@ -3982,6 +4023,35 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > > > 		vcpu->arch.guest_fpu.xfd_err = data;
> > > > 		break;
> > > > #endif
> > > > +#define CET_IBT_MASK_BITS	GENMASK_ULL(63, 2)
> > > bit9:6 are reserved even if IBT is supported.
> > Yes, as IBT is only available on Intel platforms, I move the handling of bit
> > 9:6 to VMX  related patch.
> IIUC, bits 9:6 are not reserved for IBT. I don't get how IBT availability
> affects the handling of bits 9:6.

I handle it in this way,  when IBT is not available all bits 63:2 should 
be handled as reserved. When IBT is

available, additional checks for bits 9:6 should be enforced.

> > Here's the common check in case IBT is not available.
> >
> > > > @@ -12131,6 +12217,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> > > >
> > > > 	vcpu->arch.cr3 = 0;
> > > > 	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> > > > +	memset(vcpu->arch.cet_s_ssp, 0, sizeof(vcpu->arch.cet_s_ssp));
> > > ... this begs the question: where other MSRs are reset. I suppose
> > > U_CET/PL3_SSP are handled when resetting guest FPU. But how about S_CET
> > > and INT_SSP_TAB? there is no answer in this patch.
> > I think the related guest VMCS fields(S_CET/INT_SSP_TAB/SSP) should be reset
> > to 0 in vmx_vcpu_reset(),
> >
> > do you think so?
> Yes, looks good.