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@xxxxxxxxxx> 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. 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
