Re: [PATCH] x86/altp2m: p2m_altp2m_get_or_propagate() should honor ap2m->default_access

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.02.2024 14:45, Tamas K Lengyel wrote:
> On Thu, Feb 8, 2024 at 2:46 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 08.02.2024 05:32, George Dunlap wrote:
>>> Er, ok, just one more comment: this could allow an altp2m to have more
>>> permissions than the host; for example, the host p2m entry could be
>>> p2m_access_r, but if the altp2m's default_access were p2m_access_rw,
>>> it would override that.  Is that the behavior we want?  Or do we want
>>> to do some sort of intersection of permissions?
>>>
>>> If the former, I'd propose the comment be adjusted thus:
> 
> No intersection of permissions please, that needlessly complicates
> things and makes it hard to reason about the state of a view where
> default permissions are used. No need to force a specific type of
> usecase here where the hostp2m's permissions are special just cause we
> say so. No, the permissions in the hostp2m should not have more weight
> then the specifically requested default permission.
> 
>>>
>>>  * If the entry is invalid, and the host entry was valid, propagate
>>>  * the host's entry to the altp2m, retaining page order but using the
>>>  * altp2m's default_access, and indicate that the caller should re-try
>>>  * the faulting instruction.
>>
>> I find it highly questionable that such blind overriding should be taking
>> place.
> 
> It's not blind overriding, it's the requested default permission set
> for a view where no entry was present before. It is the expected
> behavior. It would be way harder to design applications with this
> feature if it was special cased and it would take different
> permissions based on what permission is set in another view.

But the default can be only one specific value: How do you make sure that
R/O, R/X, and R/W mappings all retain their respective restrictions in the
alternative view? To not over-restrict permissions, the default would then
need to be RWX, yet then all mappings will have full permission. What am I
missing?

Jan