linux-kernel - Re: [PATCH 2/4] KVM: VMX: Introduce vmx_msr_bitmap_l01

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Date:   Sun, 12 Sep 2021 18:58:33 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] KVM: VMX: Introduce vmx_msr_bitmap_l01_changed()
 helper

On Fri, 2021-09-10 at 18:06 +0200, Vitaly Kuznetsov wrote:
> In preparation to enabling 'Enlightened MSR Bitmap' feature for Hyper-V
> guests move MSR bitmap update tracking to a dedicated helper.
> 
> Note: vmx_msr_bitmap_l01_changed() is called when MSR bitmap might be
> updated. KVM doesn't check if the bit we're trying to set is already set
> (or the bit it's trying to clear is already cleared). Such situations
> should not be common and a few false positives should not be a problem.
> 
> No functional change intended.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ae470afcb699..ad33032e8588 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3725,6 +3725,17 @@ static void vmx_set_msr_bitmap_write(ulong *msr_bitmap, u32 msr)
>  		__set_bit(msr & 0x1fff, msr_bitmap + 0xc00 / f);
>  }
>  
> +static void vmx_msr_bitmap_l01_changed(struct vcpu_vmx *vmx)
> +{
> +	/*
> +	 * When KVM is a nested hypervisor on top of Hyper-V and uses
> +	 * 'Enlightened MSR Bitmap' feature L0 needs to know that MSR
> +	 * bitmap has changed.
> +	 */
> +	if (static_branch_unlikely(&enable_evmcs))
> +		evmcs_touch_msr_bitmap();
> +}
> +
>  void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -3733,8 +3744,7 @@ void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>  	if (!cpu_has_vmx_msr_bitmap())
>  		return;
>  
> -	if (static_branch_unlikely(&enable_evmcs))
> -		evmcs_touch_msr_bitmap();
> +	vmx_msr_bitmap_l01_changed(vmx);
>  
>  	/*
>  	 * Mark the desired intercept state in shadow bitmap, this is needed
> @@ -3778,8 +3788,7 @@ void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>  	if (!cpu_has_vmx_msr_bitmap())
>  		return;
>  
> -	if (static_branch_unlikely(&enable_evmcs))
> -		evmcs_touch_msr_bitmap();
> +	vmx_msr_bitmap_l01_changed(vmx);
>  
>  	/*
>  	 * Mark the desired intercept state in shadow bitmap, this is needed
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky