linux-kernel - Re: [PATCH v2 13/15] KVM: x86/vmx: Save/Restore guest Arch LBR Ctrl msr at SMM entry/exit

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Date:   Mon, 30 Jan 2023 20:50:48 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <pbonzini@...hat.com>, <jmattson@...gle.com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <like.xu.linux@...il.com>, <kan.liang@...ux.intel.com>,
        <wei.w.wang@...el.com>
Subject: Re: [PATCH v2 13/15] KVM: x86/vmx: Save/Restore guest Arch LBR Ctrl
 msr at SMM entry/exit


On 1/28/2023 6:11 AM, Sean Christopherson wrote:
> On Thu, Nov 24, 2022, Yang Weijiang wrote:
>> Per SDM 3B Chapter 18: "IA32_LBR_CTL.LBREn is saved and cleared on #SMI,
>> and restored on RSM", store guest IA32_LBR_CTL in SMRAM and clear LBREn
>> in VMCS at SMM entry, and do reverse things at SMM exit.

[...]


>> @@ -8006,11 +8006,21 @@ static int vmx_enter_smm(struct kvm_vcpu *vcpu, union kvm_smram *smram)
>>   	vmx->nested.smm.vmxon = vmx->nested.vmxon;
>>   	vmx->nested.vmxon = false;
>>   	vmx_clear_hlt(vcpu);
>> +
>> +	if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR) &&
>> +	    guest_cpuid_has(vcpu, X86_FEATURE_LM)) {
> Uh, so this arbitrary dependency on 64-bit vCPUs needs to be factored into the
> enabling.  And KVM should WARN if arch LBRs get enabled for a 32-bit vCPU.

OK, will add the check while creating event.

>
>> +		u64 ctl = vmcs_read64(GUEST_IA32_LBR_CTL);
>> +
>> +		smram->smram64.arch_lbr_ctl = ctl;
>> +		vmcs_write64(GUEST_IA32_LBR_CTL, ctl & ~ARCH_LBR_CTL_LBREN);
>> +	}
>> +
>>   	return 0;
>>   }
>>   
>>   static int vmx_leave_smm(struct kvm_vcpu *vcpu, const union kvm_smram *smram)
>>   {
>> +	struct lbr_desc *lbr_desc = vcpu_to_lbr_desc(vcpu);
>>   	struct vcpu_vmx *vmx = to_vmx(vcpu);
>>   	int ret;
>>   
>> @@ -8027,6 +8037,18 @@ static int vmx_leave_smm(struct kvm_vcpu *vcpu, const union kvm_smram *smram)
>>   		vmx->nested.nested_run_pending = 1;
>>   		vmx->nested.smm.guest_mode = false;
>>   	}
>> +
>> +	if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR) &&
>> +	    guest_cpuid_has(vcpu, X86_FEATURE_LM)) {
>> +		u64 ctl = smram->smram64.arch_lbr_ctl;
>> +
>> +		vmcs_write64(GUEST_IA32_LBR_CTL, ctl & ARCH_LBR_CTL_LBREN);
> IIUC, this should set only LBREn and preserve all other bits, not clobber the
> entire MSR.

Oops, it's a typo, thanks!

>
>> +
>> +		if (intel_pmu_lbr_is_enabled(vcpu) &&
>> +		    (ctl & ARCH_LBR_CTL_LBREN) && !lbr_desc->event)
>> +			intel_pmu_create_guest_lbr_event(vcpu);
>> +	}
>> +
>>   	return 0;
>>   }
>>   
>> -- 
>> 2.27.0
>>