Re: [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  writes:
> 
> > Ram Pai  writes:
> >> --- a/arch/powerpc/kernel/process.c
> >> +++ b/arch/powerpc/kernel/process.c
> >> @@ -42,6 +42,7 @@
> >>  #include 
> >>  #include 
> >>  #include 
> >> +#include 
> >>
> >>  #include 
> >>  #include 
> >> @@ -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

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

[Michael Ellerman]

Thiago Jung Bauermann  writes:

> Ram Pai  writes:
>> --- a/arch/powerpc/kernel/process.c
>> +++ b/arch/powerpc/kernel/process.c
>> @@ -42,6 +42,7 @@
>>  #include 
>>  #include 
>>  #include 
>> +#include 
>>
>>  #include 
>>  #include 
>> @@ -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, 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.

cheers

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

[Thiago Jung Bauermann]

Ram Pai  writes:
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -42,6 +42,7 @@
>  #include 
>  #include 
>  #include 
> +#include 
>
>  #include 
>  #include 
> @@ -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?

> @@ -1131,6 +1139,16 @@ static inline void restore_sprs(struct thread_struct 
> *old_thread,
>   mtspr(SPRN_TAR, new_thread->tar);
>   }
>  #endif
> +#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> + if (arch_pkeys_enabled()) {
> + 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
>  }
>
>  struct task_struct *__switch_to(struct task_struct *prev,
> @@ -1689,6 +1707,13 @@ void start_thread(struct pt_regs *regs, unsigned long 
> start, unsigned long sp)
>   current->thread.tm_tfiar = 0;
>   current->thread.load_tm = 0;
>  #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
> +#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> + if (arch_pkeys_enabled()) {
> + current->thread.amr   = 0x0ul;
> + current->thread.iamr  = 0x0ul;
> + current->thread.uamor = 0x0ul;
> + }
> +#endif /* CONFIG_PPC64_MEMORY_PROTECTION_KEYS */
>  }
>  EXPORT_SYMBOL(start_thread);

-- 
Thiago Jung Bauermann
IBM Linux Technology Center