On Wed, Feb 29, 2012 at 02:44:24PM +0000, Ian Campbell wrote:

On Wed, 2012-02-29 at 12:58 +0000, Dave Martin wrote:

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On Wed, Feb 29, 2012 at 09:56:02AM +0000, Ian Campbell wrote:

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On Wed, 2012-02-29 at 09:34 +0000, Dave Martin wrote:

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On Tue, Feb 28, 2012 at 12:28:29PM +0000, Stefano Stabellini wrote:

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I don't have a very strong opinion on which register we should use, but
I would like to avoid r7 if it is already actively used by gcc.

But there is no framepointer for Thumb-2 code (?)

Peter Maydell suggested there was:

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r7 is (used by gcc as) the Thumb frame pointer; I don't know if this
makes it worth avoiding in this context.

Sounds like it might be a gcc-ism, possibly a non-default option?

Anyway, I think r12 will be fine for our purposes so the point is rather
moot.

Just had a chat with some tools guys -- apparently, when passing register
arguments to gcc inline asms there really isn't a guarantee that those
variables will be in the expected registers on entry to the inline asm.

If gcc reorders other function calls or other code around the inline asm
(which it can do, except under certain controlled situations), then
intervening code can clobber any registers in general.

Or, to summarise another way, there is no way to control which register
is used to pass something to an inline asm in general (often we get away
with this, and there are a lot of inline asms in the kernel that assume
it works, but the more you inline the more likely you are to get nasty
surprises).  There is no workaroud, except on some architectures where
special asm constraints allow specific individual registers to be
specified for operands (i386 for example).

I had assumed I just couldn't find the right syntax. Useful to know that
I couldn't find it because it doesn't exist!

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If you need a specific register, this means that you must set up that
register explicitly inside the asm if you want a guarantee that the
code will work:

	asm volatile (
		"movw	r12, %[hvc_num]\n\t"

Is gcc (or gas?) smart enough to optimise this away if it turns out that
%[hvc_num] == r12?

No, unfortunately.  Except for the information defined by the constraints,
the inline asm block is completely opaque to the compiler (except for
pasting in operands -- which is a string operation done with no knowledge
of what the text means for the assembler).

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		...
		"hvc	#0"
		:: [hvc_num] "i" (NUMBER) : "r12"
	);

Of course, if you need to set up more than about 5 or 6 registers in
this way, the doubled register footprint means that the compiler will
have to start spilling stuff to the stack.


This is the kind of problem which goes away when out-of-lining the
hvc wrapper behind a C function interface, since the ABI then provides
guarantees about how values are mershaled into and out of that code.

I don't think anything would stop gcc from clobbering an argument
register right on function entry (e..g it might move r0 to r8 and
clobber r0, for whatever reason), so that they are no longer where you
expect them to be when you hit the asm. Unlikely perhaps but no more so
than the other issues you've raised?
	
Or did you mean out-of-line as in "written in a .S file" as well as out
of line?

Yes.  Some toolchains have a concept of out-of-line assembler functions
in a .c file, but gcc doesn't -- the asm is always inline in its
immediate context, even if the containing function won't be inlined.

However, the compiler would have to be applying pretty creative
optimizations to break cases cases where an inlinable function contains,
say, nothing except for declarations, the asm() and a return statement.

I feel that the kernel implicitly relies on such things working in too
many places for breakage of that assumption to go unnoticed.

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Notwithstanding the above, even if we do make theoretically unsound
(but often true) assumptions about inline asms, ARM will be no worse
than other arches in this respect.

This is true.

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Other than serving as a reminder that inline asm is a deep can of
worms, this doesn't really give us a neat solution...

How are system calls implemented on the userspace side? I confess I
don't know what the ARM syscall ABI looks like -- is it all registers or
is some of it on the stack? It sounds like the solution ought to be
pretty similar though.

I _believe_ it's now out of line in most cases.

I'm not sure I totally understand it all, though:

http://www.eglibc.org/cgi-bin/viewvc.cgi/trunk/ports/sysdeps/unix/sysv/linux/arm/eabi/

There is an internal inline syscall wrapper INTERNAL_SYSCALL_RAW(), but
I can't see where it is used.  For Thumb code it actually just munges
registers around and calls an out-of-line function.

If I grep the disassembly of a recent EABI libc, there appear to be only
207 svc call sites, and most of them look like they are out-of-linux
wrappers, generated from the DO_CALL macro in
ports/sysdeps/unix/sysv/linux/arm/eabi/sysdep.h

That's based on a hasty reading of the code though... I'm not very
familiar with the way libc works.  (Disassembling stripped arm binaries
can also be a bit unrelieable.)

It's also worth nothing that the inline asm sycall macros which used
to exist in userspace <asm/unistd.h> are gone (at least for EABI).

Cheers
---Dave