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:   Fri, 27 Jan 2023 22:11:13 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yang Weijiang <weijiang.yang@...el.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 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.
> 
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kvm/smm.c     |  1 +
>  arch/x86/kvm/smm.h     |  3 ++-
>  arch/x86/kvm/vmx/vmx.c | 22 ++++++++++++++++++++++
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
> index a9c1c2af8d94..5987090b440f 100644
> --- a/arch/x86/kvm/smm.c
> +++ b/arch/x86/kvm/smm.c
> @@ -86,6 +86,7 @@ static void check_smram_offsets(void)
>  	CHECK_SMRAM64_OFFSET(smm_revison,		0xFEFC);
>  	CHECK_SMRAM64_OFFSET(smbase,			0xFF00);
>  	CHECK_SMRAM64_OFFSET(reserved4,			0xFF04);
> +	CHECK_SMRAM64_OFFSET(arch_lbr_ctl,		0xFF10);
>  	CHECK_SMRAM64_OFFSET(ssp,			0xFF18);
>  	CHECK_SMRAM64_OFFSET(svm_guest_pat,		0xFF20);
>  	CHECK_SMRAM64_OFFSET(svm_host_efer,		0xFF28);
> diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
> index a1cf2ac5bd78..5a6479205d91 100644
> --- a/arch/x86/kvm/smm.h
> +++ b/arch/x86/kvm/smm.h
> @@ -114,7 +114,8 @@ struct kvm_smram_state_64 {
>  	u32 reserved3[3];
>  	u32 smm_revison;
>  	u32 smbase;
> -	u32 reserved4[5];
> +	u32 reserved4[3];
> +	u64 arch_lbr_ctl;
>  
>  	/* ssp and svm_* fields below are not implemented by KVM */
>  	u64 ssp;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 6ad765ea4059..cc782233c075 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -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.

> +		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.

> +
> +		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
>