Re: [PATCH 5.15.y 1/3] KVM: nVMX: Don't use Enlightened MSR Bitmap for L3

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Mar 14, 2023 at 11:19:51AM +0200, Alexandru Matei wrote:
> From: Vitaly Kuznetsov <vkuznets@redhat.com>
> 
> 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@redhat.com>
> Message-Id: <20211129094704.326635-2-vkuznets@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

You did not sign off on this backport (or any of the backports), so I
can't take them sorry.

greg k-h