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

Paolo Bonzini
2016-07-13T15:27:50Z

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