Re: [2/5] C-SKY port: Backend implementation

[Paul Koning <paulkoning@comcast.net>]

> On Jul 25, 2018, at 12:50 AM, Jeff Law <law@redhat.com> wrote:
> 
>>>> ...
>>> It did.  See TARGET_CUSTOM_FUNCTION_DESCRIPTORS and the (relatively few)
>>> ports that define it.
>> 
>> Hmmm, I completely failed to make that connection from the docs -- the
>> whole description of that hook is pretty gibberishy and I thought it was
>> something for targets where the ABI already specifies some "standard
>> calling sequence" using descriptors (C-SKY doesn't), rather than a
>> generic alternative to executable trampolines.  Putting on my doc
>> maintainer hat briefly, I can see this needs a lot of work.  :-(
> Most likely :-)  So many things to do, so little time.
> 
> 
>> 
>> Anyway, is this required for new ports nowadays?  If so, I at least know
>> what to search for now.  At this point I couldn't say whether this would
>> do anything to fix the situation on ck801 targets where there simply
>> aren't enough spare registers available to the trampoline to both hold
>> the static link and do an indirect jump.
> It's not required, but preferred, particularly if the part has an MMU
> that can provide no-execute protections on pages in memory.  If the
> target doesn't have an mmu, then it's of marginal value.
> 
> The key advantage it has over the old trampoline implementation is that
> stacks can remain non-executable, even for Ada and nested functions.
> That's a big win from a security standpoint.

Non-executable stacks are a very good thing.

That said, I also looked at the target hook documentation and was left without any clue whatsoever.  It sure isn't clear what powers of two have to do with descriptors, or what descriptors have to do with support for nested functions.

Can you suggest places to look to get an understanding of this feature?  It sounds like the only other option is "Use the source, Luke".  Any specific targets that make a good reference implementation for this?

	paul