linux-kernel - Re: [PATCH v5 2/5] KVM: x86: Add IBPB support

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Date:   Wed, 31 Jan 2018 15:28:46 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     KarimAllah Ahmed <karahmed@...zon.de>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        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>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw@...zon.co.uk>
Subject: Re: [PATCH v5 2/5] KVM: x86: Add IBPB support

> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index d46a61b..2e4e8af 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 +3343,26 @@ 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) &&
> +		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
> +			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;

Don't you want this the other way around? That is first do the disable_intercept
and then add the 'if (is_guest_mode(vcpu))' ? Otherwise the very first
MSR write from the guest is going to hit condition above and never end
up executing the disabling of the intercept?

> +
> +		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))