Re: [PATCH] KVM : Fix read/write to IA32_FEATURE_CONTROL MSR in nested virt

[Paolo Bonzini <pbonzini@redhat.com>]

Il 04/07/2013 09:10, Gleb Natapov ha scritto:
> On Thu, Jul 04, 2013 at 09:00:09AM +0200, Paolo Bonzini wrote:
>> Il 03/07/2013 15:41, Arthur Chunqi Li ha scritto:
>>> Fix read/write to IA32_FEATURE_CONTROL MSR in nested environment.
>>> Simply return 0x5 when read and generate #GP(0) when write.
>>> Delete handling codes in vmx_set_vmx_msr() and generate #GP(0) in
>>> handle_wrmsr().
>>>
>>> Signed-off-by: Arthur Chunqi Li <yzt356@gmail.com>
>>> ---
>>>  arch/x86/kvm/vmx.c |    5 +----
>>>  1 file changed, 1 insertion(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index 260a919..e125f94 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -2277,7 +2277,7 @@ static int vmx_get_vmx_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *pdata)
>>>  
>>>  	switch (msr_index) {
>>>  	case MSR_IA32_FEATURE_CONTROL:
>>> -		*pdata = 0;
>>> +		*pdata = 0x5;
>>>  		break;
>>
>> This is not in the MSR_IA32_VMX_BASIC..MSR_IA32_VMX_TRUE_ENTRY_CTLS
>> range, so you must check nested_vmx_allowed and return 0 if it is false.
>
> Or 1?

"Return 0 from the whole function" and hence #GP(0) on reads.  The MSR
doesn't exist if VMX=SMX=0.

>> Otherwise looks good.
>>
>> Paolo
>>
>>>  	case MSR_IA32_VMX_BASIC:
>>>  		/*
>>> @@ -2356,9 +2356,6 @@ static int vmx_set_vmx_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data)
> Also this function is no longer needed. You can drop it.
> 
> And what about Nadav's patch Bandan pointed too? It is not entirely
> correct, but it is close to real HW.

I don't like that it requires a firmware change in order to use nested
VMX (at least for hypervisors that read the MSR).  "Worse emulation" and
"better emulation + new firmware" are indistiguishable from the point of
view of anyone except the firmware.

IMO there is no reason for a better emulation that no one would care
about _and_ could look like a regression when updating to a newer kernel.

Paolo

>>>  	if (!nested_vmx_allowed(vcpu))
>>>  		return 0;
>>>  
>>> -	if (msr_index == MSR_IA32_FEATURE_CONTROL)
>>> -		/* TODO: the right thing. */
>>> -		return 1;
>>>  	/*
>>>  	 * No need to treat VMX capability MSRs specially: If we don't handle
>>>  	 * them, handle_wrmsr will #GP(0), which is correct (they are readonly)
>>>
> 
> --
> 			Gleb.
>