Re: [RFC PATCH 3/4] mmu: don't set the present bit unconditionally

[Bandan Das <bsd@redhat.com>]

Xiao Guangrong <guangrong.xiao@linux.intel.com> writes:

> On 06/21/2016 11:12 AM, Bandan Das wrote:
>> To support execute only mappings on behalf of L1 hypervisors,
>> we teach set_spte to honor L1's valid XWR bits. This is only
>> if host supports EPT execute only. Use ACC_USER_MASK to signify
>> if the L1 hypervisor has the present bit set.
>>
>> Signed-off-by: Bandan Das <bsd@redhat.com>
>> ---
>>   arch/x86/kvm/mmu.c         | 11 ++++++++---
>>   arch/x86/kvm/paging_tmpl.h |  2 +-
>>   2 files changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 57d8696..3ca1a99 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -2528,7 +2528,8 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>>   	if (set_mmio_spte(vcpu, sptep, gfn, pfn, pte_access))
>>   		return 0;
>>
>> -	spte = PT_PRESENT_MASK;
>> +	if (!shadow_xonly_valid)
>> +		spte = PT_PRESENT_MASK;
>
> The xonly info can be fetched from vcpu->mmu. shadow_xonly_valid looks like
> can be dropped.

I added shadow_xonly_valid mainly for is_shadow_present_pte and since it seems
it isn't needed there, I will drop it.

>>   	if (!speculative)
>>   		spte |= shadow_accessed_mask;
>>
>> @@ -2537,8 +2538,12 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>>   	else
>>   		spte |= shadow_nx_mask;
>>
>> -	if (pte_access & ACC_USER_MASK)
>> -		spte |= shadow_user_mask;
>> +	if (pte_access & ACC_USER_MASK) {
>> +		if (shadow_xonly_valid)
>> +			spte |= PT_PRESENT_MASK;
>> +		else
>> +			spte |= shadow_user_mask;
>> +	}
>
> It can be simplified by setting shadow_user_mask to PT_PRESENT_MASK
> if ept enabled.

Ok, sounds good.