linux-kernel - Re: [PATCH 1/4] nvmx: use warn_on for buggy cases when emulating invept/invvpid

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Date:	Thu, 21 Jul 2016 15:18:13 -0400
From:	Bandan Das <bsd@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	kvm@...r.kernel.org, rkrcmar@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] nvmx: use warn_on for buggy cases when emulating invept/invvpid

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 21/07/2016 00:25, Bandan Das wrote:
>> If L1 hypervisor decides to try out something weird, alert the
>> host but only less aggressively. Also, remove the comment
>> regarding nested vpid support since it is no longer valid.
>> 
>> Signed-off-by: Bandan Das <bsd@...hat.com>
>> ---
>>  arch/x86/kvm/vmx.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 64a79f2..9fd0681 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2854,7 +2854,6 @@ static int vmx_get_vmx_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 *pdata)
>>  			vmx->nested.nested_vmx_secondary_ctls_high);
>>  		break;
>>  	case MSR_IA32_VMX_EPT_VPID_CAP:
>> -		/* Currently, no nested vpid support */
>
> This is okay.
>
>>  		*pdata = vmx->nested.nested_vmx_ept_caps |
>>  			((u64)vmx->nested.nested_vmx_vpid_caps << 32);
>>  		break;
>> @@ -7462,7 +7461,7 @@ static int handle_invept(struct kvm_vcpu *vcpu)
>>  		break;
>>  	default:
>>  		/* Trap single context invalidation invept calls */
>> -		BUG_ON(1);
>> +		WARN_ON(1);
>>  		break;
>>  	}
>>  
>> @@ -7525,7 +7524,7 @@ static int handle_invvpid(struct kvm_vcpu *vcpu)
>>  		break;
>>  	default:
>>  		/* Trap individual address invalidation invvpid calls */
>> -		BUG_ON(1);
>> +		WARN_ON(1);
>>  		break;
>>  	}
>>  
>> 
>
> These are BUGs because they are checked above:
>
>         if (!(types & (1UL << type))) {
>                 nested_vmx_failValid(vcpu,
>                                 VMXERR_INVALID_OPERAND_TO_INVEPT_INVVPID);
>                 skip_emulated_instruction(vcpu);
>                 return 1;
>         }

Ah ok, this should be sufficient I think.

> Guest-triggerable WARNs are only just a little better than
> guest-triggerable BUGs.  Guest-triggerable messages should be

Yeah, a trace isn't really necessary. We know where it's from.
BUG() can also leave the module in an unclean state and prevent
it from getting unloaded which I why I think it shouldn't be on
any path that can be guest triggered.

> rate-limited printk.
>
> I don't object to the change, but the commit message should be
> modified (or the change dropped).

I will drop it and modify the commit message accordingly.

> Paolo