Re: [PATCH 16/28] KVM: VMX: Manually recalc all MSR intercepts on userspace MSR filter change

[Xin Li <xin@zytor.com>]

On 5/29/2025 4:40 PM, Sean Christopherson wrote:
> On a userspace MSR filter change, recalculate all MSR intercepts using the
> filter-agnostic logic instead of maintaining a "shadow copy" of KVM's
> desired intercepts.  The shadow bitmaps add yet another point of failure,
> are confusing (e.g. what does "handled specially" mean!?!?), an eyesore,
> and a maintenance burden.
> 
> Given that KVM *must* be able to recalculate the correct intercepts at any
> given time, and that MSR filter updates are not hot paths, there is zero
> benefit to maintaining the shadow bitmaps.

+1

To me, this patch does simplify the logic by removing the bitmap state 
management.


Just one very minor comment below — other than that:

Reviewed-by: Xin Li (Intel) <xin@zytor.com>

> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 8f7fe04a1998..6ffa2b2b85ce 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4159,35 +4074,59 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
>   	}
>   }
>   
> -void vmx_msr_filter_changed(struct kvm_vcpu *vcpu)
> +static void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>   {
> -	struct vcpu_vmx *vmx = to_vmx(vcpu);
> -	u32 i;
> -
>   	if (!cpu_has_vmx_msr_bitmap())
>   		return;
>   
> -	/*
> -	 * Redo intercept permissions for MSRs that KVM is passing through to
> -	 * the guest.  Disabling interception will check the new MSR filter and
> -	 * ensure that KVM enables interception if usersepace wants to filter
> -	 * the MSR.  MSRs that KVM is already intercepting don't need to be
> -	 * refreshed since KVM is going to intercept them regardless of what
> -	 * userspace wants.
> -	 */
> -	for (i = 0; i < ARRAY_SIZE(vmx_possible_passthrough_msrs); i++) {
> -		u32 msr = vmx_possible_passthrough_msrs[i];
> -
> -		if (!test_bit(i, vmx->shadow_msr_intercept.read))
> -			vmx_disable_intercept_for_msr(vcpu, msr, MSR_TYPE_R);
> -
> -		if (!test_bit(i, vmx->shadow_msr_intercept.write))
> -			vmx_disable_intercept_for_msr(vcpu, msr, MSR_TYPE_W);
> +	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_TSC, MSR_TYPE_R);
> +#ifdef CONFIG_X86_64
> +	vmx_disable_intercept_for_msr(vcpu, MSR_FS_BASE, MSR_TYPE_RW);
> +	vmx_disable_intercept_for_msr(vcpu, MSR_GS_BASE, MSR_TYPE_RW);
> +	vmx_disable_intercept_for_msr(vcpu, MSR_KERNEL_GS_BASE, MSR_TYPE_RW);
> +#endif
> +	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_SYSENTER_CS, MSR_TYPE_RW);
> +	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_SYSENTER_ESP, MSR_TYPE_RW);
> +	vmx_disable_intercept_for_msr(vcpu, MSR_IA32_SYSENTER_EIP, MSR_TYPE_RW);
> +	if (kvm_cstate_in_guest(vcpu->kvm)) {
> +		vmx_disable_intercept_for_msr(vcpu, MSR_CORE_C1_RES, MSR_TYPE_R);
> +		vmx_disable_intercept_for_msr(vcpu, MSR_CORE_C3_RESIDENCY, MSR_TYPE_R);
> +		vmx_disable_intercept_for_msr(vcpu, MSR_CORE_C6_RESIDENCY, MSR_TYPE_R);
> +		vmx_disable_intercept_for_msr(vcpu, MSR_CORE_C7_RESIDENCY, MSR_TYPE_R);
>   	}
>   
>   	/* PT MSRs can be passed through iff PT is exposed to the guest. */
>   	if (vmx_pt_mode_is_host_guest())
>   		pt_update_intercept_for_msr(vcpu);
> +
> +	if (vcpu->arch.xfd_no_write_intercept)
> +		vmx_disable_intercept_for_msr(vcpu, MSR_IA32_XFD, MSR_TYPE_RW);
> +
> +
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_SPEC_CTRL, MSR_TYPE_RW,
> +				  !to_vmx(vcpu)->spec_ctrl);
> +
> +	if (kvm_cpu_cap_has(X86_FEATURE_XFD))
> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_XFD_ERR, MSR_TYPE_R,
> +					  !guest_cpu_cap_has(vcpu, X86_FEATURE_XFD));
> +
> +	if (boot_cpu_has(X86_FEATURE_IBPB))

I think Boris prefers using cpu_feature_enabled() instead — maybe this
is a good opportunity to update this occurrence?

> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PRED_CMD, MSR_TYPE_W,
> +					  !guest_has_pred_cmd_msr(vcpu));
> +
> +	if (boot_cpu_has(X86_FEATURE_FLUSH_L1D))

Ditto.

> +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
> +					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
> +
> +	/*
> +	 * x2APIC and LBR MSR intercepts are modified on-demand and cannot be
> +	 * filtered by userspace.
> +	 */
> +}