linux-kernel - Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Message-ID: <32efe0b2-d041-a70d-0e5a-8523553f068b@linux.intel.com>
Date:   Wed, 10 Jan 2018 18:47:06 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.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, ashok.raj@...el.com
Subject: Re: [PATCH 3/7] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On 01/08/2018 10:08 AM, Paolo Bonzini wrote:

> @@ -9597,6 +9620,9 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  
>  	pt_guest_enter(vmx);
>  
> +	if (have_spec_ctrl && vmx->spec_ctrl != 0)
> +		wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
> +

Say host uses IBRS (have_spec_ctrl==1), and have IBRS set to 1. 
And say guest's vmx->spec_ctrl is 0 and not using IBRS.

We will be leaving IBRS msr as 1 and won't be
switching it to 0 before entering guest.
Guest will be running with incorrect IBRS value.

Seems like the correct logic is 

if (host_supports_ibrs)
	/* switch IBRS if host and guest ibrs differs */
	if ((host_uses_ibrs && vmx->spec_ctrl == 0) ||    /* host IBRS 1, guest IBRS 0 */
	    (!host_uses_ibrs && vmx->spec_ctrl == 1))     /* host IBRS 0, guest IBRS 1 */
		wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
}

Your have_spec_ctrl logic specifies that IBRS is available.
But that doesn't necessarily mean that we will use IBRS in
host.

Tim