linux-kernel - Re: [PATCH 5/5] x86/feature: Detect the x86 feature Indirect Branch Prediction Barrier

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Date:   Fri, 12 Jan 2018 11:08:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ashok Raj <ashok.raj@...el.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>
Subject: Re: [PATCH 5/5] x86/feature: Detect the x86 feature Indirect Branch
 Prediction Barrier

On Thu, Jan 11, 2018 at 05:32:19PM -0800, Ashok Raj wrote:
> @@ -1711,11 +1715,18 @@ 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 could be recycled, causing a false negative in svm_vcpu_load;
> +     * block speculative execution.
> +     */
> +	if (boot_cpu_has(X86_FEATURE_PRED_CMD))
> +        native_wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
>  }

> @@ -3837,6 +3839,12 @@ 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 (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
> +        native_wrmsrl(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
>  }

Whitespace damage.

Also, why not introduce a helper like:

static inline flush_ibpb(void)
{
	if (static_cpu_has(X86_FEATURE_SPEC_CTRL))
		native_write_msr(MSR_IA32_PRED_CMD, FEATURE_SET_IBPB);
}

?