linux-kernel - Re: [RFC v5 38/38] Documentation: PowerPC specific updates to memory protection keys

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Date:   Sun, 9 Jul 2017 22:59:59 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
        aneesh.kumar@...ux.vnet.ibm.com, bsingharora@...il.com,
        dave.hansen@...el.com, hbabu@...ibm.com, arnd@...db.de,
        akpm@...ux-foundation.org, corbet@....net, mingo@...hat.com
Subject: Re: [RFC v5 38/38] Documentation: PowerPC specific updates to memory
 protection keys

On Mon, Jul 10, 2017 at 08:37:04AM +0530, Anshuman Khandual wrote:
> On 07/06/2017 02:52 AM, Ram Pai wrote:
> > Add documentation updates that capture PowerPC specific changes.
> > 
> > Signed-off-by: Ram Pai <linuxram@...ibm.com>
> > ---
> >  Documentation/vm/protection-keys.txt |   85 ++++++++++++++++++++++++++--------
> >  1 files changed, 65 insertions(+), 20 deletions(-)
> > 
> > diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt
> > index b643045..d50b6ab 100644
> > --- a/Documentation/vm/protection-keys.txt
> > +++ b/Documentation/vm/protection-keys.txt
> > @@ -1,21 +1,46 @@
> > -Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
> > -which will be found on future Intel CPUs.
> > +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature found in
> > +new generation of intel CPUs and on PowerPC 7 and higher CPUs.
> >  
> >  Memory Protection Keys provides a mechanism for enforcing page-based
> > -protections, but without requiring modification of the page tables
> > -when an application changes protection domains.  It works by
> > -dedicating 4 previously ignored bits in each page table entry to a
> > -"protection key", giving 16 possible keys.
> > -
> > -There is also a new user-accessible register (PKRU) with two separate
> > -bits (Access Disable and Write Disable) for each key.  Being a CPU
> > -register, PKRU is inherently thread-local, potentially giving each
> > -thread a different set of protections from every other thread.
> > -
> > -There are two new instructions (RDPKRU/WRPKRU) for reading and writing
> > -to the new register.  The feature is only available in 64-bit mode,
> > -even though there is theoretically space in the PAE PTEs.  These
> > -permissions are enforced on data access only and have no effect on
> > +protections, but without requiring modification of the page tables when an
> > +application changes protection domains.
> > +
> > +
> > +On Intel:
> > +
> > +	It works by dedicating 4 previously ignored bits in each page table
> > +	entry to a "protection key", giving 16 possible keys.
> > +
> > +	There is also a new user-accessible register (PKRU) with two separate
> > +	bits (Access Disable and Write Disable) for each key.  Being a CPU
> > +	register, PKRU is inherently thread-local, potentially giving each
> > +	thread a different set of protections from every other thread.
> > +
> > +	There are two new instructions (RDPKRU/WRPKRU) for reading and writing
> > +	to the new register.  The feature is only available in 64-bit mode,
> > +	even though there is theoretically space in the PAE PTEs.  These
> > +	permissions are enforced on data access only and have no effect on
> > +	instruction fetches.
> > +
> > +
> > +On PowerPC:
> > +
> > +	It works by dedicating 5 page table entry bits to a "protection key",
> > +	giving 32 possible keys.
> > +
> > +	There  is  a  user-accessible  register (AMR)  with  two separate bits;
> > +	Access Disable and  Write  Disable, for  each key.  Being  a  CPU
> > +	register,  AMR  is inherently  thread-local,  potentially  giving  each
> > +	thread a different set of protections from every other thread.  NOTE:
> > +	Disabling read permission does not disable write and vice-versa.
> 
> We can only enable/disable entire access or write. Then how
> read permission can be changed with protection keys directly ?

Good catch. On powerpc there is a disable read and disable write. They
both can be combined to disable access. Will fix the error. Read it
as 'Access Read' . thanks.

RP