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

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

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

The one who wants to sleep well at night ;-)

>> 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;

(void *) usually smells a bit fishy to me but it seems to fit here.

>
> 		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
>> 
>

-- 
Vitaly