Re: [PATCH v2 4/5] mmu: change unconditional setting of the u bit in fault bitmap

[Paolo Bonzini]

On 13/07/2016 10:23, Paolo Bonzini wrote:
> 
> 
> On 13/07/2016 00:18, Bandan Das wrote:
>> For the nested EPT case, we assume that the read bit (u) is
>> always set since we used to unconditionally set it in set_spte().
>> Modify it to only be set when host ept execute only support
>> isn't present.
>>
>> Signed-off-by: Bandan Das <bsd@xxxxxxxxxx>
>> ---
>>  arch/x86/kvm/mmu.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index c364dcb..566eea5 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -3923,9 +3923,10 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
>>  				 *   clearer.
>>  				 */
>>  				smap = cr4_smap && u && !uf && !ff;
>> -			} else
>> -				/* Not really needed: no U/S accesses on ept  */
>> -				u = 1;
>> +			} else {
>> +				if (shadow_present_mask)
>> +					u = 1;

... in fact I think this should be removed completely.

If shadow_present_mask is set, the R bit will always be set in a present
PTE and FNAME(gpte_access) will change that to ACC_USER_MASK---which in
turn means that u=1.

Paolo

>> +			}
>>  
>>  			fault = (ff && !x) || (uf && !u) || (wf && !w) ||
>>  				(smapf && smap);
>>
> 
> This is needed too, in order to look up with the correct uf:
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 003618e324ce..941d345ebd2c 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6014,12 +6014,14 @@ 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;
> 
>  	vcpu->arch.exit_qualification = exit_qualification;
> 
> 
> Otherwise you always lookup the permission bitmap for an uf=0 case.
> This was the meaning of the "Not really needed" comment.
> 
> Paolo
>