linux-kernel - Re: [PATCH 05/15] KVM: VMX: Add document to state that write to uret msr should always be intercepted

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Date:   Thu, 18 Nov 2021 17:05:22 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 05/15] KVM: VMX: Add document to state that write to uret
 msr should always be intercepted

On 11/18/21 12:08, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> And adds a corresponding sanity check code.
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index e8a41fdc3c4d..cd081219b668 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3703,13 +3703,21 @@ void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>   	if (!cpu_has_vmx_msr_bitmap())
>   		return;
>   
> +	/*
> +	 * Write to uret msr should always be intercepted due to the mechanism
> +	 * must know the current value.  Santity check to avoid any inadvertent
> +	 * mistake in coding.
> +	 */
> +	if (WARN_ON_ONCE(vmx_find_uret_msr(vmx, msr) && (type & MSR_TYPE_W)))
> +		return;
> +

I'm not sure about this one, it's relatively expensive to call 
vmx_find_uret_msr.

User-return MSRs and disable-intercept MSRs are almost the opposite: 
uret is for MSRs that the host (not even the processor) never uses, 
disable-intercept is for MSRs that the guest reads/writes often.  As 
such it seems almost impossible that they overlap.

Paolo