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

Bandan Das
2016-07-13T15:47:45Z

Paolo Bonzini <pbonzini@redhat.com> writes:

> 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.

shadow_present_mask actually signifies different things for ept/non-ept
cases and it doesn't hurt to mention it. But I get your point, maybe,
it's self-explanatory.

>>> 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.

What I meant is how they are the same bit. is_shadow_present_pte() is probably one
and another one is link_shadow_page() which already has a BUILD_BUG_ON().

> Paolo