Re: [RFC PATCH 0/4] Add support for EPT execute only for nested hypervisors

[Bandan Das <bsd@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 24/06/2016 06:50, Bandan Das wrote:
>>>> >> I am tempted to remove the FNAME version altogether and change
>>>> >> is_present_gpte()
>>>> >> to return (pte & PT_PRESENT_MASK) || (shadow_xonly_valid && (pte & 4)). This
>>>> >> will take care of all cases. Hope I am not missing something :)
>>> >
>>> > Please rename the non-FNAME version to pae_is_present_pdpte or just inline
>>> > it in the two callers.
>> I am still not sure why the FNAME version is needed, specifically the PTTYPE_EPT
>> specific check. Why can't we  just check for execonly and the corresponding
>> bit in the non FNAME version ?
>
> The FNAME version encodes the right semantics:
>
> 1) for non-EPT page tables, bit 1 indicates present.
>
> 2) for EPT page tables, setting any bit 0-2 indicates present.
>
> It's perfectly fine to find that a PTE is invalid *after* judging that
> it is present.  There is no difference between setting bit 51 on a PTE,
> and having the invalid -W- combination on an EPT page table entry.  In
> both cases, the page is present but the PTE is invalid.
>
> The SDM is very clear about it: "If bits 2:0 of an EPT paging-structure
> entry are all 0, the entry is not present".  So if bits 2:0 are -W- the
> entry is present but invalid, and causes an EPT misconfiguration vmexit.
>  A non-present entry will cause an EPT violation instead.

Thank you very much, this clears it. pte & 7 makes sense now.

Xiao, as Paolo suggested, I can just remove the non FNAME version and directly
do pte & PT_PRESENT_MASK at the two call sites. I hope that will future proof it.

Thanks,
Bandan

> Paolo