Re: [PATCH v2 0/5] Add support for EPT execute only for nested hypervisors

From: Paolo Bonzini <pbonzini@redhat.com>
To: Bandan Das <bsd@redhat.com>
Cc: kvm@vger.kernel.org, guangrong.xiao@linux.intel.com,
	kernellwp@gmail.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/5] Add support for EPT execute only for nested hypervisors
Date: Wed, 13 Jul 2016 17:27:05 +0200	[thread overview]
Message-ID: <921eef54-f23b-cd90-8e20-a428a00a3297@redhat.com> (raw)
In-Reply-To: <jpgwpkpwp9x.fsf@linux.bootlegged.copy>

On 13/07/2016 17:06, Bandan Das wrote:
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 190c0559c221..bd2535fdb9eb 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -2524,11 +2524,10 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>>  		return 0;
>>  
>>  	/*
>> -	 * In the non-EPT case, execonly is not valid and so
>> -	 * the following line is equivalent to spte |= PT_PRESENT_MASK.
>>  	 * For the EPT case, shadow_present_mask is 0 if hardware
>> -	 * supports it and we honor whatever way the guest set it.
>> -	 * See: FNAME(gpte_access) in paging_tmpl.h
>> +	 * supports exec-only page table entries.  In that case,
>> +	 * ACC_USER_MASK and shadow_user_mask are used to represent
>> +	 * read access.  See FNAME(gpte_access) in paging_tmpl.h.
>>  	 */
> 
> I would still prefer a note about the non-EPT case, makes it easy to
> understand.

I can add "shadow_present_mask is PT_PRESENT_MASK in the non-EPT case"
but it's a bit of a tautology.

>>  	spte |= shadow_present_mask;
>>  	if (!speculative)
>> @@ -3923,9 +3922,6 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
>>  				 *   clearer.
>>  				 */
>>  				smap = cr4_smap && u && !uf && !ff;
>> -			} else {
>> -				if (shadow_present_mask)
>> -					u = 1;
>>  			}
>>  
>>  			fault = (ff && !x) || (uf && !u) || (wf && !w) ||
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 576c47cda1a3..dfef081e76c0 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -6120,12 +6120,14 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
>>  	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
>>  	trace_kvm_page_fault(gpa, exit_qualification);
>>  
>> -	/* It is a write fault? */
>> +	/* it is a read fault? */
>> +	error_code = (exit_qualification << 2) & PFERR_USER_MASK;
>> +	/* it is a write fault? */
>>  	error_code = exit_qualification & PFERR_WRITE_MASK;
>>  	/* It is a fetch fault? */
>>  	error_code |= (exit_qualification << 2) & PFERR_FETCH_MASK;
>>  	/* ept page table is present? */
>> -	error_code |= (exit_qualification >> 3) & PFERR_PRESENT_MASK;
>> +	error_code |= (exit_qualification & 0x38) != 0;
>>
> 
> Thank you for the thorough review here. I missed that we didn't set the read bit
> at all. I am still a little unclear how permission_fault works though...
> 
>>  	vcpu->arch.exit_qualification = exit_qualification;
>>  
>> @@ -6474,8 +6476,7 @@ static __init int hardware_setup(void)
>>  			(enable_ept_ad_bits) ? VMX_EPT_DIRTY_BIT : 0ull,
>>  			0ull, VMX_EPT_EXECUTABLE_MASK,
>>  			cpu_has_vmx_ept_execute_only() ?
>> -				      0ull : PT_PRESENT_MASK);
>> -		BUILD_BUG_ON(PT_PRESENT_MASK != VMX_EPT_READABLE_MASK);
>> +				      0ull : VMX_EPT_READABLE_MASK);
> 
> I wanted to keep it the former way because "PT_PRESENT_MASK is equal to VMX_EPT_READABLE_MASK"
> is an assumption all throughout. I wanted to use this section to catch mismatches.

I think there's no such assumption anymore, actually.  Can you double
check?  If there are any, that's where the BUILD_BUG_ON should be.

Paolo