On Fri, 2021-11-05 at 00:52 +0000, Sean Christopherson wrote:
> On Wed, Oct 13, 2021, Vitaly Kuznetsov wrote:
> > 3-level nesting is also not a very common setup nowadays.
>
> Says who? :-D
I regularly test 4 level nesting :P
It's KVM all the way down....
But jokes aside 3 level nesting will start to happen occasionally more and more often,
IMHO with windows guests which have accidently/or on purpose enabled HypoerV/Core isolation/WSL3 inside,
and that are run nested on KVM.
Just FYI. I have a patch series pending (reviews are welcome!) which implement nested vVMLOAD/vVMSAVE and
vGIF which allows L1 to use these optional SVM features to run its nested guests (that is L3s) faster.
(This series is the reason I was recently stress testing 3/4 level nesting.
4 levels usually work so slow that VM doesn't boot and timeouts in various systemd settings).
3rd level works not that bad IMHO.
All that said I don't have any objections to the patch itself.
Best regards,
Maxim Levitsky
>
> > Don't enable 'Enlightened MSR Bitmap' feature for KVM's L2s (real L3s) for
> > now.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/vmx/vmx.c | 21 ++++++++++++---------
> > 1 file changed, 12 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 1c8b2b6e7ed9..e82cdde58119 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -2655,15 +2655,6 @@ 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));
> > @@ -6903,6 +6894,18 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
> >
> > vmx->loaded_vmcs = &vmx->vmcs01;
> >
> > + /*
> > + * Use Hyper-V 'Enlightened MSR Bitmap' feature when KVM runs as a
> > + * nested (L1) hypervisor and Hyper-V in L0 supports it.
>
> And maybe call out specifically that KVM intentionally uses this only for vmcs02?
>
> > + */
> > + if (IS_ENABLED(CONFIG_HYPERV) && static_branch_unlikely(&enable_evmcs)
> > + && (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {
>
> && on the previous line, I think we'll survive the 82 char line :-)
>
> > + struct hv_enlightened_vmcs *evmcs =
> > + (struct hv_enlightened_vmcs *)vmx->loaded_vmcs->vmcs;
>
> Hmm, what about landing this right after vmcs01's VMCS is allocated? It's kinda
> weird, but it makes it more obvious that ->vmcs is not NULL. And if the cast is
> simply via a "void *" it all fits on one line.
>
> err = alloc_loaded_vmcs(&vmx->vmcs01);
> 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 an
> * enlightened bitmap 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;
> }
>
> > +
> > + evmcs->hv_enlightenments_control.msr_bitmap = 1;
> > + }
> > +
> > if (cpu_need_virtualize_apic_accesses(vcpu)) {
> > err = alloc_apic_access_page(vcpu->kvm);
> > if (err)
> > --
> > 2.31.1
> >
