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

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Date:   Fri, 16 Mar 2018 12:31:52 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Ingo Molnar <mingo@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>, linux-mm <linux-mm@...ck.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
        Haren Myneni/Beaverton/IBM <hbabu@...ibm.com>,
        Michal Hocko <mhocko@...nel.org>,
        Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>, fweimer@...hat.com,
        msuchanek@...e.com, Thomas Gleixner <tglx@...utronix.de>,
        Ulrich.Weigand@...ibm.com, Ram Pai <ram.n.pai@...il.com>
Subject: Re: [PATCH v4] mm, pkey: treat pkey-0 special

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@...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.

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