linux-kernel - Re: [PATCH 4/7] kvm: vmx: Set IBPB when running a different VCPU

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Date:   Mon, 08 Jan 2018 21:23:51 +0200
From:   Liran Alon <LIRAN.ALON@...CLE.COM>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
CC:     jmattson@...gle.com, aliguori@...zon.com, thomas.lendacky@....com,
        dwmw@...zon.co.uk, bp@...en8.de,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH 4/7] kvm: vmx: Set IBPB when running a different VCPU



On 08/01/18 20:08, Paolo Bonzini wrote:
> From: Tim Chen <tim.c.chen@...ux.intel.com>
>
> Ensure an IBPB (Indirect branch prediction barrier) before every VCPU
> switch.
>
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>   arch/x86/kvm/vmx.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index d00bcad7336e..bf127c570675 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -2375,6 +2375,8 @@ 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);
> +		if (have_spec_ctrl)
> +			wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
>   	}
>
>   	if (!already_loaded) {
> @@ -4029,6 +4031,13 @@ static void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
>   	free_vmcs(loaded_vmcs->vmcs);
>   	loaded_vmcs->vmcs = NULL;
>   	WARN_ON(loaded_vmcs->shadow_vmcs != NULL);
> +
> +	/*
> +	 * The VMCS could be recycled, causing a false negative in
> +	 * vmx_vcpu_load; block speculative execution.
> +	 */
> +	if (have_spec_ctrl)
> +		wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
>   }
>
>   static void vmx_nested_free_vmcs02(struct vcpu_vmx *vmx)
>

Reviewed-by: Liran Alon <liran.alon@...cle.com>