linux-kernel - Re: [PATCH 3/5] mmu: don't set the present bit unconditionally

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Date:	Tue, 28 Jun 2016 22:21:56 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Bandan Das <bsd@...hat.com>
Cc:	kvm@...r.kernel.org, guangrong.xiao@...ux.intel.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] mmu: don't set the present bit unconditionally



On 28/06/2016 19:30, Bandan Das wrote:
> Paolo Bonzini <pbonzini@...hat.com> writes:
>> On 28/06/2016 06:32, Bandan Das wrote:
>>> +	bool execonly = !(context->guest_rsvd_check.bad_mt_xwr &
>>> +			  (1ull << VMX_EPT_EXECUTABLE_MASK));
>>>  
>>>  	if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
>>>  		return 0;
>>>  
>>> -	spte = PT_PRESENT_MASK;
>>> +	if (!execonly)
>>> +		spte |= PT_PRESENT_MASK;
>>
>> This needs a comment:
>>
>> 	/*
>> 	 * There are two cases in which execonly is false: 1) for
>> 	 * non-EPT page tables, in which case we need to set the
>> 	 * P bit; 2) for EPT page tables where an X-- page table
>> 	 * entry is invalid, in which case we need to force the R
>> 	 * bit of the page table entry to 1.
>> 	 */
> 
> I think this should be: 2) for EPT page tables where an X-- page
> table entry is invalid and a EPT misconfig is injected to the guest
> before we reach here.

Right, I messed this one up.  shadow_user_mask and ACC_USER_MASK work
the same way on processors that do not support execonly.

Paolo

>> 	BUILD_BUG_ON(PT_PRESENT_MASK != VMX_EPT_READABLE_MASK);
>> 	if (!execonly)
>> 		spte |= PT_PRESENT_MASK;
>> 	
>>
>>>  	if (!speculative)
>>>  		spte |= shadow_accessed_mask;
>>>
>>>  	if (enable_ept) {
>>> -		kvm_mmu_set_mask_ptes(0ull,
>>> +		kvm_mmu_set_mask_ptes(PT_PRESENT_MASK,
>>
>> This should be VMX_EPT_READABLE_MASK.
>>
>>> @@ -2531,6 +2535,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>>>  	else
>>>  		spte |= shadow_nx_mask;
>>>  
>>> +	/* In the EPT case, shadow_user_mask is PT_PRESENT_MASK */
>>
>> I don't think this comment is necessary, but it's better to add one in
>> FNAME(gpte_access).
>>
>> 	/*
>> 	 * In the EPT case, a page table can be executable but not
>> 	 * readable (on some processors).  Therefore, set_spte does not
>> 	 * automatically set bit 0 if execute-only is supported.
>> 	 * Instead, since EPT page tables do not have a U bit, we
>> 	 * repurpose ACC_USER_MASK to signify readability.  Likewise,
>> 	 * when EPT is in use shadow_user_mask is set to
>> 	 * VMX_EPT_READABLE_MASK.
>> 	 */
>> 	
>>
>> Thanks,
>>
>> Paolo
>>
>>>  	if (pte_access & ACC_USER_MASK)
>>>  		spte |= shadow_user_mask;
>>
>>
>> Paolo
>>
>>>  			(enable_ept_ad_bits) ? VMX_EPT_ACCESS_BIT : 0ull,
>>>  			(enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
>>>  			0ull, VMX_EPT_EXECUTABLE_MASK);
>>
>> --
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>