Date: Sat, 29 Jul 2017 16:31:13 -0700
From: Ram Pai <linuxram@us.ibm.com>
To: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
        arnd@arndb.de, corbet@lwn.net, mhocko@kernel.org,
        dave.hansen@intel.com, mingo@redhat.com, paulus@samba.org,
        aneesh.kumar@linux.vnet.ibm.com, akpm@linux-foundation.org,
        khandual@linux.vnet.ibm.com
Subject: Re: [RFC v6 20/62] powerpc: store and restore the pkey state across
 context switches
Reply-To: Ram Pai <linuxram@us.ibm.com>
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On Thu, Jul 27, 2017 at 02:32:59PM -0300, Thiago Jung Bauermann wrote:
> 
> Ram Pai <linuxram@us.ibm.com> writes:
> 
> > Store and restore the AMR, IAMR and UMOR register state of the task
> > before scheduling out and after scheduling in, respectively.
> >
> > Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> 
> s/UMOR/UAMOR/
> 
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index 2ad725e..9429361 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -1096,6 +1096,11 @@ static inline void save_sprs(struct thread_struct *t)
> >  		t->tar = mfspr(SPRN_TAR);
> >  	}
> >  #endif
> > +#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> > +	t->amr = mfspr(SPRN_AMR);
> > +	t->iamr = mfspr(SPRN_IAMR);
> > +	t->uamor = mfspr(SPRN_UAMOR);
> > +#endif
> >  }
> >
> >  static inline void restore_sprs(struct thread_struct *old_thread,
> > @@ -1131,6 +1136,14 @@ static inline void restore_sprs(struct thread_struct *old_thread,
> >  			mtspr(SPRN_TAR, new_thread->tar);
> >  	}
> >  #endif
> > +#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> > +	if (old_thread->amr != new_thread->amr)
> > +		mtspr(SPRN_AMR, new_thread->amr);
> > +	if (old_thread->iamr != new_thread->iamr)
> > +		mtspr(SPRN_IAMR, new_thread->iamr);
> > +	if (old_thread->uamor != new_thread->uamor)
> > +		mtspr(SPRN_UAMOR, new_thread->uamor);
> > +#endif
> >  }
> 
> Shouldn't the saving and restoring of the SPRs be guarded by a check for
> whether memory protection keys are enabled? What happens when trying to
> access these registers on a CPU which doesn't have them?

Good point. need to guard it.  However; i think, these registers have been
available since power6.

RP