linux-kernel - Re: [PATCH v2 31/32] KVM: x86: Simplify userspace filter logic when disabling MSR interception

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Message-ID: <8f71ad94-034c-4282-85eb-abedda49ece4@linux.intel.com>
Date: Wed, 11 Jun 2025 10:35:26 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Chao Gao <chao.gao@...el.com>, Borislav Petkov <bp@...en8.de>,
 Xin Li <xin@...or.com>, Francesco Lavra <francescolavra.fl@...il.com>,
 Manali Shukla <Manali.Shukla@....com>
Subject: Re: [PATCH v2 31/32] KVM: x86: Simplify userspace filter logic when
 disabling MSR interception


On 6/11/2025 6:57 AM, Sean Christopherson wrote:
> Refactor {svm,vmx}_disable_intercept_for_msr() to simplify the handling of
> userspace filters that disallow access to an MSR.  The more complicated
> logic is no longer needed or justified now that KVM recalculates all MSR
> intercepts on a userspace MSR filter change, i.e. now that KVM doesn't
> need to also update shadow bitmaps.
>
> No functional change intended.
>
> Suggested-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/svm.c | 24 ++++++++++--------------
>  arch/x86/kvm/vmx/vmx.c | 24 ++++++++++--------------
>  2 files changed, 20 insertions(+), 28 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index e3c49c763225..5453478d1ca3 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -691,24 +691,20 @@ void svm_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>  	void *msrpm = svm->msrpm;
>  
>  	/* Don't disable interception for MSRs userspace wants to handle. */
> -	if ((type & MSR_TYPE_R) &&
> -	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ)) {
> -		svm_set_msr_bitmap_read(msrpm, msr);
> -		type &= ~MSR_TYPE_R;
> +	if (type & MSR_TYPE_R) {
> +		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ))
> +			svm_clear_msr_bitmap_read(msrpm, msr);
> +		else
> +			svm_set_msr_bitmap_read(msrpm, msr);
>  	}
>  
> -	if ((type & MSR_TYPE_W) &&
> -	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE)) {
> -		svm_set_msr_bitmap_write(msrpm, msr);
> -		type &= ~MSR_TYPE_W;
> +	if (type & MSR_TYPE_W) {
> +		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE))
> +			svm_clear_msr_bitmap_write(msrpm, msr);
> +		else
> +			svm_set_msr_bitmap_write(msrpm, msr);
>  	}
>  
> -	if (type & MSR_TYPE_R)
> -		svm_clear_msr_bitmap_read(msrpm, msr);
> -
> -	if (type & MSR_TYPE_W)
> -		svm_clear_msr_bitmap_write(msrpm, msr);
> -
>  	svm_hv_vmcb_dirty_nested_enlightenments(vcpu);
>  	svm->nested.force_msr_bitmap_recalc = true;
>  }
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index bdff81f8288d..277c6b5b5d5f 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3962,23 +3962,19 @@ void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
>  
>  	vmx_msr_bitmap_l01_changed(vmx);
>  
> -	if ((type & MSR_TYPE_R) &&
> -	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ)) {
> -		vmx_set_msr_bitmap_read(msr_bitmap, msr);
> -		type &= ~MSR_TYPE_R;
> +	if (type & MSR_TYPE_R) {
> +		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_READ))
> +			vmx_clear_msr_bitmap_read(msr_bitmap, msr);
> +		else
> +			vmx_set_msr_bitmap_read(msr_bitmap, msr);
>  	}
>  
> -	if ((type & MSR_TYPE_W) &&
> -	    !kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE)) {
> -		vmx_set_msr_bitmap_write(msr_bitmap, msr);
> -		type &= ~MSR_TYPE_W;
> +	if (type & MSR_TYPE_W) {
> +		if (kvm_msr_allowed(vcpu, msr, KVM_MSR_FILTER_WRITE))
> +			vmx_clear_msr_bitmap_write(msr_bitmap, msr);
> +		else
> +			vmx_set_msr_bitmap_write(msr_bitmap, msr);
>  	}
> -
> -	if (type & MSR_TYPE_R)
> -		vmx_clear_msr_bitmap_read(msr_bitmap, msr);
> -
> -	if (type & MSR_TYPE_W)
> -		vmx_clear_msr_bitmap_write(msr_bitmap, msr);
>  }
>  
>  void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)

Reviewed-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>