linux-kernel - Re: [PATCH 1/4] KVM: MMU: fix permission

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Date:	Fri, 25 Mar 2016 21:41:59 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: MMU: fix permission_fault()



On 03/25/2016 09:35 PM, Paolo Bonzini wrote:
>
>
> On 25/03/2016 14:19, Xiao Guangrong wrote:
>> kvm-unit-tests complained about the PFEC is not set properly, e.g,:
>> test pte.rw pte.d pte.nx pde.p pde.rw pde.pse user fetch: FAIL: error code 15
>> expected 5
>> Dump mapping: address: 0x123400000000
>> ------L4: 3e95007
>> ------L3: 3e96007
>> ------L2: 2000083
>
> What's the command line for the reproducer?

QEMU=/home/eric/qemu/x86_64-softmmu/qemu-system-x86_64 ./x86-run x86/access.flat

>
>> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
>> index b70df72..81bffd1 100644
>> --- a/arch/x86/kvm/mmu.h
>> +++ b/arch/x86/kvm/mmu.h
>> @@ -154,7 +154,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>>   				  unsigned pfec)
>>   {
>>   	int cpl = kvm_x86_ops->get_cpl(vcpu);
>> -	unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
>> +	unsigned long errcode, rflags = kvm_x86_ops->get_rflags(vcpu);
>>
>>   	/*
>>   	 * If CPL < 3, SMAP prevention are disabled if EFLAGS.AC = 1.
>> @@ -175,7 +175,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>>   	bool fault = (mmu->permissions[index] >> pte_access) & 1;
>>
>>   	WARN_ON(pfec & (PFERR_PK_MASK | PFERR_RSVD_MASK));
>> -	pfec |= PFERR_PRESENT_MASK;
>> +	errcode = PFERR_PRESENT_MASK;
>
> So is this patch doing the same as "KVM: MMU: precompute page fault
> error code"?  It was necessary after all. :)

Sorry for my mistake... I missed the logic you changed :(

I still prefer to calculating the error code on the fault path which is rare, or
think a way to encapsulate it to permission_fault()...