linux-kernel - Re: [PATCH 6/8] kvm: svm: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Date:   Wed, 10 Jan 2018 14:13:28 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, liran.alon@...cle.com, jmattson@...gle.com,
        aliguori@...zon.com, dwmw@...zon.co.uk, bp@...en8.de,
        x86@...nel.org
Subject: Re: [PATCH 6/8] kvm: svm: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to guest

On 1/9/2018 6:03 AM, Paolo Bonzini wrote:
> Direct access to MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD is important
> for performance.  Allow load/store of MSR_IA32_SPEC_CTRL, restore guest
> IBRS on VM entry and set it to 0 on VM exit (because Linux does not use
> it yet).
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/svm.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 31ace8d7774a..934a21e02e03 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c

...

> @@ -5015,6 +5045,18 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>  #endif
>  		);
>  
> +	if (have_spec_ctrl) {
> +		rdmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
> +		if (svm->spec_ctrl != 0)
> +			wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> +	}
> +	/*
> +	 * Speculative execution past the above wrmsrl might encounter
> +	 * an indirect branch and use guest-controlled contents of the
> +	 * indirect branch predictor; block it.
> +	 */
> +	asm("lfence");

This will end up needing to be an alternative macro based on the
LFENCE_RDTSC or MFENCE_RDTSC features [1].  You'll probably just want to
use the speculation barrier macro that ends up being defined to control
the speculation here.

Thanks,
Tom

[1] https://marc.info/?l=linux-kernel&m=151545930207815&w=2

> +
>  #ifdef CONFIG_X86_64
>  	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
>  #else
>