This is a note to let you know that I've just added the patch titled
KVM: nVMX: Don't use Enlightened MSR Bitmap for L3
to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kvm-nvmx-don-t-use-enlightened-msr-bitmap-for-l3.patch
and it can be found in the queue-5.10 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 250552b925ce400c17d166422fde9bb215958481 Mon Sep 17 00:00:00 2001
From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Date: Mon, 29 Nov 2021 10:47:01 +0100
Subject: KVM: nVMX: Don't use Enlightened MSR Bitmap for L3
From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
commit 250552b925ce400c17d166422fde9bb215958481 upstream.
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: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Message-Id: <20211129094704.326635-2-vkuznets@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Alexandru Matei <alexandru.matei@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kvm/vmx/vmx.c | 22 +++++++++++++---------
1 file changed, 13 insertions(+), 9 deletions(-)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2725,15 +2725,6 @@ int alloc_loaded_vmcs(struct 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));
@@ -7029,6 +7020,19 @@ static int vmx_create_vcpu(struct kvm_vc
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);
Patches currently in stable-queue which might be from vkuznets@xxxxxxxxxx are
queue-5.10/kvm-vmx-introduce-vmx_msr_bitmap_l01_changed-helper.patch
queue-5.10/kvm-nvmx-don-t-use-enlightened-msr-bitmap-for-l3.patch
