linux-kernel - Re: [PATCH v10 22/27] KVM: VMX: Set up interception for CET MSRs

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Date: Mon, 6 May 2024 16:48:54 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <pbonzini@...hat.com>, <dave.hansen@...el.com>, <x86@...nel.org>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peterz@...radead.org>, <chao.gao@...el.com>, <rick.p.edgecombe@...el.com>,
	<mlevitsk@...hat.com>, <john.allen@....com>
Subject: Re: [PATCH v10 22/27] KVM: VMX: Set up interception for CET MSRs

On 5/2/2024 7:07 AM, Sean Christopherson wrote:
> On Sun, Feb 18, 2024, Yang Weijiang wrote:
>> @@ -7767,6 +7771,41 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>>   		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>>   }
>>   
>> +static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
>> +{
>> +	bool incpt;
>> +
>> +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
>> +		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>> +
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
>> +					  MSR_TYPE_RW, incpt);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
>> +					  MSR_TYPE_RW, incpt);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
>> +					  MSR_TYPE_RW, incpt);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
>> +					  MSR_TYPE_RW, incpt);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
>> +					  MSR_TYPE_RW, incpt);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
>> +					  MSR_TYPE_RW, incpt);
>> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
>> +					  MSR_TYPE_RW, incpt);
>> +		if (!incpt)
>> +			return;
> Hmm, I find this is unnecessarily confusing and brittle.  E.g. in the unlikely
> event more CET stuff comes along, this lurking return could cause problems.
>
> Why not handle S_CET and U_CET in a single common path?  IMO, this is less error
> prone, and more clearly captures the relationship between S/U_CET, SHSTK, and IBT.
> Updating MSR intercepts is not a hot path, so the overhead of checking guest CPUID
> multiple times should be a non-issue.  And eventually KVM should effectively cache
> all of those lookups, i.e. the cost will be negilible.
>
> 	bool incpt;
>
> 	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> 		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>
> 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> 					  MSR_TYPE_RW, incpt);
> 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> 					  MSR_TYPE_RW, incpt);
> 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> 					  MSR_TYPE_RW, incpt);
> 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
> 					  MSR_TYPE_RW, incpt);
> 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> 					  MSR_TYPE_RW, incpt);
> 	}
>
> 	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK) ||
> 	    kvm_cpu_cap_has(X86_FEATURE_IBT)) {
> 		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT) &&
> 			!guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>
> 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> 					  MSR_TYPE_RW, incpt);
> 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> 					  MSR_TYPE_RW, incpt);
> 	}

It looks fine to me, will apply it, thanks!