linux-kernel - Re: [PATCH v2 1/3] KVM: vmx: fix ept reserved bits for 1-GByte page

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Date:	Tue, 19 Aug 2014 19:16:22 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...nel.org>,
	Zhang Yang <yang.z.zhang@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: vmx: fix ept reserved bits for 1-GByte page

On Tue, Aug 19, 2014 at 11:09:49AM +0200, Paolo Bonzini wrote:
>Il 19/08/2014 11:04, Wanpeng Li ha scritto:
>> EPT misconfig handler in kvm will check which reason lead to EPT 
>> misconfiguration after vmexit. One of the reasons is that an EPT 
>> paging-structure entry is configured with settings reserved for 
>> future functionality. However, the handler can't identify if 
>> paging-structure entry of reserved bits for 1-GByte page are 
>> configured, since PDPTE which point to 1-GByte page will reserve 
>> bits 29:12 instead of bits 7:3 which are reserved for PDPTE that 
>> references an EPT Page Directory. This patch fix it by reserve 
>> bits 29:12 for 1-GByte page. 
>> 
>> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
>> ---
>> v1 -> v2:
>>  * same "if" statement cover both 2MB and 1GB pages
>>  * return 0xf8 for level == 4
>
>I think you dropped this check by mistake.

Indeed. I will do it in next version.

>
>>  * get the level by checking the return value of ept_rsvd_mask 
>> 
>>  arch/x86/kvm/vmx.c | 19 +++++++------------
>>  1 file changed, 7 insertions(+), 12 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index cad37d5..2763f37 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -5521,17 +5521,12 @@ static u64 ept_rsvd_mask(u64 spte, int level)
>>  	for (i = 51; i > boot_cpu_data.x86_phys_bits; i--)
>>  		mask |= (1ULL << i);
>>  
>> -	if (level > 2)
>> -		/* bits 7:3 reserved */
>> -		mask |= 0xf8;
>> -	else if (level == 2) {
>> -		if (spte & (1ULL << 7))
>> -			/* 2MB ref, bits 20:12 reserved */
>> -			mask |= 0x1ff000;
>> -		else
>> -			/* bits 6:3 reserved */
>> -			mask |= 0x78;
>> -	}
>> +	if (spte & (1ULL << 7))
>
>You need to go this way if level == 1 too.  Otherwise, you would report
>bits 6:3 reserved if the hypervisor is using the ignored bit 7 (Table
>28-6, Format of an EPT Page-Table Entry).
>

Agreed. What still need to do here is to update the comments in order to  
include level == 1, right?

>> +		/* 1GB/2MB page, bits 29:12 or 20:12 reserved respectively */
>> +		mask |= (PAGE_SIZE << ((level - 1) * 9)) - PAGE_SIZE;
>> +	else
>> +		/* bits 6:3 reserved */
>> +		mask |= 0x78;
>>  
>>  	return mask;
>>  }
>> @@ -5561,7 +5556,7 @@ static void ept_misconfig_inspect_spte(struct kvm_vcpu *vcpu, u64 spte,
>>  			WARN_ON(1);
>>  		}
>>  
>> -		if (level == 1 || (level == 2 && (spte & (1ULL << 7)))) {
>> +		if (level == 1 || (rsvd_bits & 0x38)) {
>
>- rsvd_bits will always be zero here.  You need to check the return
>value of ept_rsvd_mask().  Let's call it rsvd_mask in the rest of this
>email.
>
>- the test is inverted, you need to check that bits 5:3 are _not_
>reserved, hence (rsvd_mask & 0x38) == 0.
>
>- once you do this, the test also covers level 1.

Agreed.

>
>I suggest that you write a testcase for kvm-unit-tests.
>

Ok. 

Regards,
Wanpeng Li 

>Paolo
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