linux-kernel - Re: [PATCH v14 11/13] KVM: VMX: Pass through CET MSRs to the guest when supported

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Message-ID: <e383a377-ee64-342d-b1dd-0f99186714e3@redhat.com>
Date:   Thu, 28 Jan 2021 19:04:06 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Weijiang <weijiang.yang@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, sean.j.christopherson@...el.com,
        jmattson@...gle.com
Cc:     yu.c.zhang@...ux.intel.com
Subject: Re: [PATCH v14 11/13] KVM: VMX: Pass through CET MSRs to the guest
 when supported

On 28/01/21 18:54, Paolo Bonzini wrote:
> On 06/11/20 02:16, Yang Weijiang wrote:
>> Pass through all CET MSRs when the associated CET component (kernel vs.
>> user) is enabled to improve guest performance.  All CET MSRs are context
>> switched, either via dedicated VMCS fields or XSAVES.
>>
>> Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
>> Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
>> Co-developed-by: Sean Christopherson <sean.j.christopherson@...el.com>
>> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 29 +++++++++++++++++++++++++++++
>>   1 file changed, 29 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index c88a6e1721b1..6ba2027a3d44 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -7366,6 +7366,32 @@ static void update_intel_pt_cfg(struct kvm_vcpu 
>> *vcpu)
>>           vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>>   }
>> +static bool is_cet_state_supported(struct kvm_vcpu *vcpu, u32 xss_state)
>> +{
>> +    return (vcpu->arch.guest_supported_xss & xss_state) &&
>> +           (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
>> +        guest_cpuid_has(vcpu, X86_FEATURE_IBT));
>> +}
>> +
>> +static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
>> +{
>> +    bool incpt = !is_cet_state_supported(vcpu, XFEATURE_MASK_CET_USER);
>> +
>> +    vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET, MSR_TYPE_RW, incpt);
>> +
>> +    incpt |= !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>> +    vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, 
>> incpt);
>> +
>> +    incpt = !is_cet_state_supported(vcpu, XFEATURE_MASK_CET_KERNEL);
>> +    vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, incpt);
>> +
>> +    incpt |= !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>> +    vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, 
>> 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);
>> +}
>> +
>>   static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>>   {
>>       struct vcpu_vmx *vmx = to_vmx(vcpu);
>> @@ -7409,6 +7435,9 @@ static void vmx_vcpu_after_set_cpuid(struct 
>> kvm_vcpu *vcpu)
>>       /* Refresh #PF interception to account for MAXPHYADDR changes. */
>>       update_exception_bitmap(vcpu);
>> +
>> +    if (kvm_cet_supported())
>> +        vmx_update_intercept_for_cet_msr(vcpu);
>>   }
>>   static __init void vmx_set_cpu_caps(void)
>>
> 
> Can you do this only if CR4.CET=1?

Actually, considering this is XSAVES and not RDMSR/WRMSR state, this is 
okay as is.

Paolo