[RFC v7 09/25] powerpc: store and restore the pkey state across context switches

[Ram Pai]

On Fri, Aug 11, 2017 at 04:34:19PM +1000, Michael Ellerman wrote:
> Thiago Jung Bauermann <bauerman at linux.vnet.ibm.com> writes:
> 
> > Ram Pai <linuxram at us.ibm.com> writes:
> >> --- a/arch/powerpc/kernel/process.c
> >> +++ b/arch/powerpc/kernel/process.c
> >> @@ -42,6 +42,7 @@
> >>  #include <linux/hw_breakpoint.h>
> >>  #include <linux/uaccess.h>
> >>  #include <linux/elf-randomize.h>
> >> +#include <linux/pkeys.h>
> >>
> >>  #include <asm/pgtable.h>
> >>  #include <asm/io.h>
> >> @@ -1096,6 +1097,13 @@ static inline void save_sprs(struct thread_struct *t)
> >>  		t->tar = mfspr(SPRN_TAR);
> >>  	}
> >>  #endif
> >> +#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> >> +	if (arch_pkeys_enabled()) {
> >> +		t->amr = mfspr(SPRN_AMR);
> >> +		t->iamr = mfspr(SPRN_IAMR);
> >> +		t->uamor = mfspr(SPRN_UAMOR);
> >> +	}
> >> +#endif
> >>  }
> >
> > Is it worth having a flag in thread_struct saying whether it has every
> > called pkey_alloc and only do the mfsprs if it did?

Yes. This will further optimize the code; a great thing!

> 
> Yes, in fact there's a programming note in the UAMOR section of the arch
> that says exactly that.
> 
> On the write side you have to be a bit more careful. You have to make
> sure you set the UAMOR to 0 when you're switching from a process that
> has used keys to one that isn't.

Currently we save and restore AMR/IAMR/UAMOR if the OS has enabled pkeys.
This means the UAMOR will get restored to 0 if the application has not
used any keys.

But if we do optimize the code further; as suggested by Thiago, we will
have to be careful with initializing UAMOR while switching back task
that has not used the keys yet.

RP

> 
> cheers

-- 
Ram Pai