Re: [PATCH] KVM/VMX: Check ept_pointer before flushing ept tlb

[Tianyu Lan <ltykernel@gmail.com>]

Hi Vitaly:
	Thanks for your review.

On 11/6/2018 11:50 PM, Vitaly Kuznetsov wrote:
> ltykernel@gmail.com writes:
> 
>> From: Lan Tianyu <Tianyu.Lan@microsoft.com>
>>
>> This patch is to initialize ept_pointer to INVALID_PAGE and check it
>> before flushing ept tlb. If ept_pointer is invalidated, bypass the flush
>> request.
>>
>> Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
>> ---
>>   arch/x86/kvm/vmx.c | 16 +++++++++++++---
>>   1 file changed, 13 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 4555077d69ce..edbc96cb990a 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -1580,14 +1580,22 @@ static int vmx_hv_remote_flush_tlb(struct kvm *kvm)
>>   	/*
>>   	 * FLUSH_GUEST_PHYSICAL_ADDRESS_SPACE hypercall needs the address of the
>>   	 * base of EPT PML4 table, strip off EPT configuration information.
>> +	 * If ept_pointer is invalid pointer, bypass the flush request.
>>   	 */
>>   	if (to_kvm_vmx(kvm)->ept_pointers_match != EPT_POINTERS_MATCH) {
>> -		kvm_for_each_vcpu(i, vcpu, kvm)
>> +		kvm_for_each_vcpu(i, vcpu, kvm) {
>> +			if (!VALID_PAGE(to_vmx(vcpu)->ept_pointer))
>> +				return 0;
>> +
> 
> To be honest I fail to understand the reason behind the patch: instead
> of doing one unneeded flush request with ept_pointer==0 (after vCPU is
> initialized) we now do the check every time. Could you please elaborate
> on why this is needed?

The reason to introduce the check here is to avoid flushing ept tlb
without valid ept table. When nested guest boots up and only BP is
active, we should not do flush for APs and L1 hypervisor hasn't set
valid EPT table for APs.

> 
>>   			ret |= hyperv_flush_guest_mapping(
>> -				to_vmx(kvm_get_vcpu(kvm, i))->ept_pointer & PAGE_MASK);
>> +				to_vmx(vcpu)->ept_pointer & PAGE_MASK);
> 
> I would use a local variable for 'to_vmx(vcpu)->ept_pointer' or even
> 'to_vmx(vcpu)->ept_pointer & PAGE_MASK' and use it in VALID_PAGE() - as
> lower bits are unrelated;

Yes, that makes sense. INVALID_PAGE also contains lower bits and so a 
local variable for 'to_vmx(vcpu)->ept_pointer' maybe better.


> 
> 
>> +		}
>>   	} else {
>> +		if (!VALID_PAGE(to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer))
>> +			return 0;
> 
> Ditto.
> 
>> +
>>   		ret = hyperv_flush_guest_mapping(
>> -				to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer & PAGE_MASK);
>> +			to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer & PAGE_MASK);
> 
> This doesn't belong to this patch.

I found the line exceeds 80 chars and so adjust indent. Maybe I should 
change it in a separate patch despite it's a small change.

> 
>>   	}
>>   
>>   	spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
>> @@ -11568,6 +11576,8 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
>>   	vmx->pi_desc.nv = POSTED_INTR_VECTOR;
>>   	vmx->pi_desc.sn = 1;
>>   
>> +	vmx->ept_pointer = INVALID_PAGE;
>> +
>>   	return &vmx->vcpu;
>>   
>>   free_vmcs:
>