Re: [RFC v6 21/62] powerpc: introduce execute-only pkey

[Ram Pai]

On Thu, Aug 17, 2017 at 04:35:55PM -0700, Ram Pai wrote:
> On Wed, Aug 02, 2017 at 07:40:46PM +1000, Michael Ellerman wrote:
> > Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxxxxxxx> writes:
> > 
> > > Michael Ellerman <mpe@xxxxxxxxxxxxxx> writes:
> > >
> > >> Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxxxxxxx> writes:
> > >>> Ram Pai <linuxram@xxxxxxxxxx> writes:
> > >> ...
> > >>>> +
> > >>>> +	/* We got one, store it and use it from here on out */
> > >>>> +	if (need_to_set_mm_pkey)
> > >>>> +		mm->context.execute_only_pkey = execute_only_pkey;
> > >>>> +	return execute_only_pkey;
> > >>>> +}
> > >>>
> > >>> If you follow the code flow in __execute_only_pkey, the AMR and UAMOR
> > >>> are read 3 times in total, and AMR is written twice. IAMR is read and
> > >>> written twice. Since they are SPRs and access to them is slow (or isn't
> > >>> it?),
> > >>
> > >> SPRs read/writes are slow, but they're not *that* slow in comparison to
> > >> a system call (which I think is where this code is being called?).
> > >
> > > Yes, this code runs on mprotect and mmap syscalls if the memory is
> > > requested to have execute but not read nor write permissions.
> > 
> > Yep. That's not in the fast path for key usage, ie. the fast path is
> > userspace changing the AMR itself, and the overhead of a syscall is
> > already hundreds of cycles.
> > 
> > >> So we should try to avoid too many SPR read/writes, but at the same time
> > >> we can accept more than the minimum if it makes the code much easier to
> > >> follow.
> > >
> > > Ok. Ram had asked me to suggest a way to optimize the SPR reads and
> > > writes and I came up with the patch below. Do you think it's worth it?
> > 
> > At a glance no I don't think it is. Sorry you spent that much time on it.
> > 
> > I think we can probably reduce the number of SPR accesses without
> > needing to go to that level of complexity.
> > 
> > But don't throw the patch away, I may eat my words once I have the full
> > series applied and am looking at it hard - at the moment I'm just
> > reviewing the patches piecemeal as I get time.
> 

Thiago's patch does save some cycles. I dont feel like throwing his
work. I agree, It should be considered after applying all the patches. 
 
RP

-- 
Ram Pai