Re: [PATCH v3 1/4] KVM: nVMX: Don't use Enlightened MSR Bitmap for L3

Sean Christopherson
2021-11-05T00:52:21Z

On Wed, Oct 13, 2021, Vitaly Kuznetsov wrote:
> 3-level nesting is also not a very common setup nowadays.

Says who? :-D

> Don't enable 'Enlightened MSR Bitmap' feature for KVM's L2s (real L3s) for
> now.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
> 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
>