Commit 250552b9 authored by Vitaly Kuznetsov's avatar Vitaly Kuznetsov Committed by Paolo Bonzini
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KVM: nVMX: Don't use Enlightened MSR Bitmap for L3 



When KVM runs as a nested hypervisor on top of Hyper-V it uses Enlightened
VMCS and enables Enlightened MSR Bitmap feature for its L1s and L2s (which
are actually L2s and L3s from Hyper-V's perspective). When MSR bitmap is
updated, KVM has to reset HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP from
clean fields to make Hyper-V aware of the change. For KVM's L1s, this is
done in vmx_disable_intercept_for_msr()/vmx_enable_intercept_for_msr().
MSR bitmap for L2 is build in nested_vmx_prepare_msr_bitmap() by blending
MSR bitmap for L1 and L1's idea of MSR bitmap for L2. KVM, however, doesn't
check if the resulting bitmap is different and never cleans
HV_VMX_ENLIGHTENED_CLEAN_FIELD_MSR_BITMAP in eVMCS02. This is incorrect and
may result in Hyper-V missing the update.

The issue could've been solved by calling evmcs_touch_msr_bitmap() for
eVMCS02 from nested_vmx_prepare_msr_bitmap() unconditionally but doing so
would not give any performance benefits (compared to not using Enlightened
MSR Bitmap at all). 3-level nesting is also not a very common setup
nowadays.

Don't enable 'Enlightened MSR Bitmap' feature for KVM's L2s (real L3s) for
now.

Signed-off-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20211129094704.326635-2-vkuznets@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 0fcfb00b

arch/x86/kvm/vmx/vmx.c

+13 −9
Original line number Diff line number Diff line
@@ -2646,15 +2646,6 @@ int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs) 
		if (!loaded_vmcs->msr_bitmap)

			goto out_vmcs;

		memset(loaded_vmcs->msr_bitmap, 0xff, PAGE_SIZE);



		if (IS_ENABLED(CONFIG_HYPERV) &&

		    static_branch_unlikely(&enable_evmcs) &&

		    (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {

			struct hv_enlightened_vmcs *evmcs =

				(struct hv_enlightened_vmcs *)loaded_vmcs->vmcs;



			evmcs->hv_enlightenments_control.msr_bitmap = 1;

		}

	}



	memset(&loaded_vmcs->host_state, 0, sizeof(struct vmcs_host_state));
@@ -6842,6 +6833,19 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu) 
	if (err < 0)

		goto free_pml;



	/*

	 * Use Hyper-V 'Enlightened MSR Bitmap' feature when KVM runs as a

	 * nested (L1) hypervisor and Hyper-V in L0 supports it. Enable the

	 * feature only for vmcs01, KVM currently isn't equipped to realize any

	 * performance benefits from enabling it for vmcs02.

	 */

	if (IS_ENABLED(CONFIG_HYPERV) && static_branch_unlikely(&enable_evmcs) &&

	    (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {

		struct hv_enlightened_vmcs *evmcs = (void *)vmx->vmcs01.vmcs;



		evmcs->hv_enlightenments_control.msr_bitmap = 1;

	}



	/* The MSR bitmap starts with all ones */

	bitmap_fill(vmx->shadow_msr_intercept.read, MAX_POSSIBLE_PASSTHROUGH_MSRS);

	bitmap_fill(vmx->shadow_msr_intercept.write, MAX_POSSIBLE_PASSTHROUGH_MSRS);