[PATCH 0/4] KVM: nVMX: Enlightened MSR Bitmap feature for Hyper-V on KVM

From: Vitaly Kuznetsov
Date: Fri Sep 10 2021 - 12:07:47 EST

Updating MSR bitmap for L2 is not cheap and rearly needed. TLFS for Hyper-V
offers 'Enlightened MSR Bitmap' feature which allows L1 hypervisor to
inform L0 when it changes MSR bitmap, this eliminates the need to examine
L1's MSR bitmap for L2 every time when 'real' MSR bitmap for L2 gets

When the feature is enabled for Win10+WSL2, it shaves off around 700 CPU
cycles from a nested vmexit cost (tight cpuid loop test).

First patch of the series is unrelated to the newly implemented feature,
it fixes a bug in Enlightened MSR Bitmap usage when KVM runs as a nested
hypervisor on top of Hyper-V.

Vitaly Kuznetsov (4):
KVM: nVMX: Don't use Enlightened MSR Bitmap for L3
KVM: VMX: Introduce vmx_msr_bitmap_l01_changed() helper
KVM: nVMX: Track whether changes in L0 require MSR bitmap for L2 to be
KVM: nVMX: Implement Enlightened MSR Bitmap feature

arch/x86/kvm/hyperv.c | 2 ++
arch/x86/kvm/vmx/nested.c | 28 +++++++++++++++++++++++---
arch/x86/kvm/vmx/vmx.c | 41 ++++++++++++++++++++++++++-------------
arch/x86/kvm/vmx/vmx.h | 6 ++++++
4 files changed, 61 insertions(+), 16 deletions(-)