linux-kernel - Re: [PATCH 06/26] KVM: x86: Move MSR_IA32_BNDCFGS existence checks into vendor code

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Message-ID: <878slhkruu.fsf@vitty.brq.redhat.com>
Date:   Wed, 05 Feb 2020 15:53:45 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 06/26] KVM: x86: Move MSR_IA32_BNDCFGS existence checks into vendor code

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Move the MSR_IA32_BNDCFGS existence check into vendor code by way of
> ->has_virtualized_msr().  AMD does not support MPX, and given that Intel
> is in the process of removing MPX, it's extremely unlikely AMD will ever
> support MPX.
>
> Note, invoking ->has_virtualized_msr() requires an extra retpoline, but
> kvm_init_msr_list() is not a hot path.  As alluded to above, the
> motivation is to quarantine MPX as much as possible.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/svm.c     | 2 ++
>  arch/x86/kvm/vmx/vmx.c | 2 ++
>  arch/x86/kvm/x86.c     | 4 ----
>  3 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 4c8427f57b71..504118c49f46 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -5990,6 +5990,8 @@ static bool svm_has_virtualized_msr(u32 index)
>  	switch (index) {
>  	case MSR_TSC_AUX:
>  		return boot_cpu_has(X86_FEATURE_RDTSCP);
> +	case MSR_IA32_BNDCFGS:
> +		return false;
>  	default:
>  		break;
>  	}
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9588914e941e..dbeef64f7409 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6279,6 +6279,8 @@ static bool vmx_has_virtualized_msr(u32 index)
>  	switch (index) {
>  	case MSR_TSC_AUX:
>  		return cpu_has_vmx_rdtscp();
> +	case MSR_IA32_BNDCFGS:
> +		return kvm_mpx_supported();
>  	default:
>  		break;
>  	}
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a8619c52ea86..70cbb9164088 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5237,10 +5237,6 @@ static void kvm_init_msr_list(void)
>  		 * to the guests in some cases.
>  		 */
>  		switch (msr_index) {
> -		case MSR_IA32_BNDCFGS:
> -			if (!kvm_mpx_supported())
> -				continue;
> -			break;
>  		case MSR_IA32_RTIT_CTL:
>  		case MSR_IA32_RTIT_STATUS:
>  			if (!kvm_x86_ops->pt_supported())

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly