linux-kernel - Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

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Date:   Wed, 31 Jan 2018 12:32:41 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     KarimAllah Ahmed <karahmed@...zon.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v4 2/5] KVM: x86: Add IBPB support

Looking at SVM now...

On 31/01/2018 08:10, KarimAllah Ahmed wrote:
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index f40d0da..89495cf 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -529,6 +529,7 @@ struct svm_cpu_data {
>  	struct kvm_ldttss_desc *tss_desc;
>  
>  	struct page *save_area;
> +	struct vmcb *current_vmcb;
>  };
>  
>  static DEFINE_PER_CPU(struct svm_cpu_data *, svm_data);
> @@ -1703,11 +1704,17 @@ static void svm_free_vcpu(struct kvm_vcpu *vcpu)
>  	__free_pages(virt_to_page(svm->nested.msrpm), MSRPM_ALLOC_ORDER);
>  	kvm_vcpu_uninit(vcpu);
>  	kmem_cache_free(kvm_vcpu_cache, svm);
> +	/*
> +	 * The vmcb page can be recycled, causing a false negative in
> +	 * svm_vcpu_load(). So do a full IBPB now.
> +	 */
> +	indirect_branch_prediction_barrier();
>  }
>  
>  static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> +	struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
>  	int i;
>  
>  	if (unlikely(cpu != vcpu->cpu)) {
> @@ -1736,6 +1743,10 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	if (static_cpu_has(X86_FEATURE_RDTSCP))
>  		wrmsrl(MSR_TSC_AUX, svm->tsc_aux);
>  
> +	if (sd->current_vmcb != svm->vmcb) {
> +		sd->current_vmcb = svm->vmcb;
> +		indirect_branch_prediction_barrier();
> +	}
>  	avic_vcpu_load(vcpu, cpu);
>  }
>  
> @@ -3684,6 +3695,22 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>  	case MSR_IA32_TSC:
>  		kvm_write_tsc(vcpu, msr);
>  		break;
> +	case MSR_IA32_PRED_CMD:
> +		if (!msr->host_initiated &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_IBPB))
> +			return 1;
> +
> +		if (data & ~PRED_CMD_IBPB)
> +			return 1;
> +
> +		if (!data)
> +			break;
> +
> +		wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);

This is missing an

        { .index = MSR_IA32_PRED_CMD,                   .always = false },

in the direct_access_msrs array.  Once you do this, nested is taken care of
automatically.

Likewise for MSR_IA32_SPEC_CTRL in patch 5.

Paolo

> +		if (is_guest_mode(vcpu))
> +			break;
> +		set_msr_interception(svm->msrpm, MSR_IA32_PRED_CMD, 0, 1);
> +		break;
>  	case MSR_STAR:
>  		svm->vmcb->save.star = data;
>  		break;
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index d46a61b..96e672e 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2285,6 +2285,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	if (per_cpu(current_vmcs, cpu) != vmx->loaded_vmcs->vmcs) {
>  		per_cpu(current_vmcs, cpu) = vmx->loaded_vmcs->vmcs;
>  		vmcs_load(vmx->loaded_vmcs->vmcs);
> +		indirect_branch_prediction_barrier();
>  	}
>  
>  	if (!already_loaded) {
> @@ -3342,6 +3343,25 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	case MSR_IA32_TSC:
>  		kvm_write_tsc(vcpu, msr_info);
>  		break;
> +	case MSR_IA32_PRED_CMD:
> +		if (!msr_info->host_initiated &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_IBPB))
> +			return 1;
> +
> +		if (data & ~PRED_CMD_IBPB)
> +			return 1;
> +
> +		if (!data)
> +			break;
> +
> +		wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> +
> +		if (is_guest_mode(vcpu))
> +			break;
> +
> +		vmx_disable_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_IA32_PRED_CMD,
> +					      MSR_TYPE_W);
> +		break;
>  	case MSR_IA32_CR_PAT:
>  		if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {
>  			if (!kvm_mtrr_valid(vcpu, MSR_IA32_CR_PAT, data))
> @@ -10046,7 +10066,8 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
>  	unsigned long *msr_bitmap_l0 = to_vmx(vcpu)->nested.vmcs02.msr_bitmap;
>  
>  	/* This shortcut is ok because we support only x2APIC MSRs so far. */
> -	if (!nested_cpu_has_virt_x2apic_mode(vmcs12))
> +	if (!nested_cpu_has_virt_x2apic_mode(vmcs12) &&
> +	    !to_vmx(vcpu)->save_spec_ctrl_on_exit)
>  		return false;
>  
>  	page = kvm_vcpu_gpa_to_page(vcpu, vmcs12->msr_bitmap);
> @@ -10079,6 +10100,14 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
>  				MSR_TYPE_W);
>  		}
>  	}
> +
> +	if (to_vmx(vcpu)->save_spec_ctrl_on_exit) {
> +		nested_vmx_disable_intercept_for_msr(
> +				msr_bitmap_l1, msr_bitmap_l0,
> +				MSR_IA32_PRED_CMD,
> +				MSR_TYPE_R);
> +	}
> +
>  	kunmap(page);
>  	kvm_release_page_clean(page);
>  
>