Re: [PATCH v4] mm, pkey: treat pkey-0 special

[Balbir Singh <bsingharora@gmail.com>]

On Tue, Mar 27, 2018 at 2:48 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> Ram Pai <linuxram@us.ibm.com> writes:
>
>> On Fri, Mar 16, 2018 at 10:02:22PM +1100, Balbir Singh wrote:
>>> On Fri, Mar 16, 2018 at 9:33 PM, Ram Pai <linuxram@us.ibm.com> wrote:
>>> > Applications need the ability to associate an address-range with some
>>> > key and latter revert to its initial default key. Pkey-0 comes close to
>>> > providing this function but falls short, because the current
>>> > implementation disallows applications to explicitly associate pkey-0 to
>>> > the address range.
>>> >
>>> > Clarify the semantics of pkey-0 and provide the corresponding
>>> > implementation.
>>> >
>>> > Pkey-0 is special with the following semantics.
>>> > (a) it is implicitly allocated and can never be freed. It always exists.
>>> > (b) it is the default key assigned to any address-range.
>>> > (c) it can be explicitly associated with any address-range.
>>> >
>>> > Tested on powerpc only. Could not test on x86.
>>>
>>> Ram,
>>>
>>> I was wondering if we should check the AMOR values on the ppc side to make sure
>>> that pkey0 is indeed available for use as default. I am still of the
>>> opinion that we
>>
>> AMOR cannot be read/written by the OS in priviledge-non-hypervisor-mode.
>> We could try testing if key-0 is available to the OS by temproarily
>> changing the bits key-0 bits of AMR or IAMR register. But will be
>> dangeorous to do, for you might disable read,execute of all the pages,
>> since all pages are asscoiated with key-0 bydefault.
>
> No we should do what firmware tells us. If it says key 0 is available we
> use it, otherwise we don't.
>
> Now if you notice the way the firmware API (device tree property) is
> defined, it tells us how many keys are available, counting from 0.
>

I could not find counting from 0 anywhere, are we expected to look
at the AMOR and figure out what we have access to? Why do we
assume they'll be contiguous, it makes our life easy, but I really
could not find any documentation on it

> So for pkey 0 to be reserved there must be 0 keys available.
>
> End of story.
>
> cheers

Cheers,
Balbir Singh.