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

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Message-ID: <ZMEjudsdr8WEiw3b@chao-email>
Date:   Wed, 26 Jul 2023 21:46:33 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     "Yang, Weijiang" <weijiang.yang@...el.com>
CC:     <seanjc@...gle.com>, <pbonzini@...hat.com>, <peterz@...radead.org>,
        <john.allen@....com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <rick.p.edgecombe@...el.com>,
        <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v4 09/20] KVM:x86: Add common code of CET MSR access

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.,

	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).

>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.

>
>> > 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.

>
>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.