[RFC 0/4] Create infrastructure for running C code from SRAM.

[Dave.Martin@arm.com (Dave Martin)]

On Fri, Sep 06, 2013 at 12:12:21PM +0100, Russell King - ARM Linux wrote:
> On Tue, Sep 03, 2013 at 09:44:21AM -0700, Russ Dill wrote:
> > SRAM handling code is in the process of being moved from arch directories
> > into drivers/misc/sram.c using device tree and genalloc [1] [2]. This RFC
> > patchset builds on that, including the limitation that the SRAM address is
> > not known at compile time. Because the SRAM address is not known at compile
> > time, the code that runs from SRAM must be compiled with -fPIC. Even if
> > the code were loaded to a fixed virtual address, portions of the code must
> > often be run with the MMU disabled.
> 
> What are you doing about the various gcc utility functions that may be
> implicitly called from C code such as memcpy and memset?
> 
> > The general idea is that for each SRAM user (such as an SoC specific
> > suspend/resume mechanism) to create a group of sections. The section group
> > is created with a single macro for each user, but end up looking like this:
> > 
> > .sram.am33xx : AT(ADDR(.sram.am33xx) - 0) {
> >   __sram_am33xx_start = .;
> >   *(.sram.am33xx.*)
> >   __sram_am33xx_end = .;
> > }
> > 
> > Any data or functions that should be copied to SRAM for this use should be
> > maked with an appropriate __section() attribute. A helper is then added for
> > translating between the original kernel symbol, and the address of that
> > function or variable once it has been copied into SRAM. Once control is
> > passed to a function within the SRAM section grouping, it can access any
> > variables or functions within that same SRAM section grouping without
> > translation.
> 
> What about the relocations which will need to be fixed up - eg, addresses
> in the literal pool, the GOT table contents, etc?  You say nothing about
> this.

I was also thinking about this, and there are more problems.

As well as what has already been mentioned:

 * Calls from inside the SRAM code to vmlinux (including lib1funcs etc.)
   will typically break, except on architectures where function calls are
   (absolute by default not ARM).

 * The compiler/linker won't detect unsafe constructs or code generation,
   because it assumes that anything built with -fPIC is going to be patched
   up later by ld.so or equivalent.

 * The GOT is generated by the linker, and is a single table.  Yet each
   SRAM blob needs to be able to refer to its own GOT entries position-
   independently.  Moving the blobs independently won't work.

In other words: -fPIC does not generate position-independent code.

It generates position-dependent code that is easier to move around than
non-fPIC code, but you still need a dynamic linker (or equivalent) to
make it all work.


There are various "correct" ways to handle this, the simplest of which
is probably to build each SRAM blob as a kernel module, embed the result
in the kernel somehow, and then use the module loader infrastructure
to handle fixing the module up to the right address.

But this is still likely to be overkill, given the small scale of the
SRAM code.

Restricting such code to carefully-written assembler (as now) may be
the more practical approrach, unless there's a good example of somewhere
that C code would provide a big benefit.

Cheers
---Dave