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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Maxim Levitsky <mlevitsk@redhat.com> writes:

> On Fri, 2021-09-10 at 18:06 +0200, Vitaly Kuznetsov wrote:
>> 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>
>> ---
>>  arch/x86/kvm/vmx/vmx.c | 22 +++++++++++++---------
>>  1 file changed, 13 insertions(+), 9 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 0c2c0d5ae873..ae470afcb699 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -2654,15 +2654,6 @@ 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));
>> @@ -6861,6 +6852,19 @@ 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.
>> +	 */
>> +	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 *)vmx->loaded_vmcs->vmcs;
>> +
>> +		evmcs->hv_enlightenments_control.msr_bitmap = 1;
>> +	}
>> +
>>  	cpu = get_cpu();
>>  	vmx_vcpu_load(vcpu, cpu);
>>  	vcpu->cpu = cpu;
>
> Makes sense.
>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
>
>
> However, just a note that it is very very confusing that KVM can use eVMCS in both ways.
>  
>  
> 'Client': It can both run under HyperV, and thus take advantage of eVMCS when it runs its guests (with
> help of
> HyperV)
>  
> 'Server' KVM can emulate some HyperV features, and one of these is eVMCS, thus a windows guest running
> under KVM, can use KVM's eVMCS implementation to run nested guests.
>  
> This patch fails under
> 'Client', while the other patches in the series fall under the 'Server' category,
> and even more confusing, the patch 2 moves 'Client' code around, but it is intended for following patches
> 3,4 which are
> for Server.
>  

All this is confusing indeed, KVM-on-Hyper-V and Hyper-V-on-KVM are two
different beasts but it's not always clear from patch subject. I was
thinking about adding this to patch prexes:

"KVM: VMX: KVM-on-Hyper-V: ... " 
"KVM: nVMX: Hyper-V-on-KVM ..."

or something similar.

>
> Thus this patch probably should be a separate patch, just to avoid confusion.
>

This patch is a weird one. We actually fix

Hyper-V-on-KVM-on-Hyper-V case.

Don't get confused! :-)


> However, since this patch series is already posted, and I figured that out, and hopefully explained it here,
> no need to do anything though!
>
>
> Best regards,
> 	Maxim Levitsky
>
>
>

-- 
Vitaly