On Fri, Mar 16, 2018 at 10:02:22PM +1100, Balbir Singh wrote:

On Fri, Mar 16, 2018 at 9:33 PM, Ram Pai [off-list ref] wrote:

quoted

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.

May be we can play with UAMOR register and check if its key-0 can be
modified. That is a good indication that key-0 is available.
If it is not available, disable the pkey-subsystem, and operate
the legacy way; no pkeys.

should consider non-0 default pkey in the long run. I'm OK with the patches for
now, but really 0 is not special except for it being the default bit
values present
in the PTE.

it will be a pain. Any new pte that gets instantiated will now have to
explicitly initialize its key to this default-non-zero-key.  I hope
we or any architecture goes there ever.

-- 
Ram Pai