bpf: Use r constraint instead of p constraint in selftests

Some of the BPF selftests use the "p" constraint in inline assembly
snippets, for input operands for MOV (rN = rM) instructions.

This is mainly done via the __imm_ptr macro defined in
tools/testing/selftests/bpf/progs/bpf_misc.h:

  #define __imm_ptr(name) [name]"p"(&name)

Example:

  int consume_first_item_only(void *ctx)
  {
        struct bpf_iter_num iter;
        asm volatile (
                /* create iterator */
                "r1 = %[iter];"
                [...]
                :
                : __imm_ptr(iter)
                : CLOBBERS);
        [...]
  }

The "p" constraint is a tricky one.  It is documented in the GCC manual
section "Simple Constraints":

  An operand that is a valid memory address is allowed.  This is for
  ``load address'' and ``push address'' instructions.

  p in the constraint must be accompanied by address_operand as the
  predicate in the match_operand.  This predicate interprets the mode
  specified in the match_operand as the mode of the memory reference for
  which the address would be valid.

There are two problems:

1. It is questionable whether that constraint was ever intended to be
   used in inline assembly templates, because its behavior really
   depends on compiler internals.  A "memory address" is not the same
   than a "memory operand" or a "memory reference" (constraint "m"), and
   in fact its usage in the template above results in an error in both
   x86_64-linux-gnu and bpf-unkonwn-none:

     foo.c: In function ‘bar’:
     foo.c:6:3: error: invalid 'asm': invalid expression as operand
        6 |   asm volatile ("r1 = %[jorl]" : : [jorl]"p"(&jorl));
          |   ^~~

   I would assume the same happens with aarch64, riscv, and most/all
   other targets in GCC, that do not accept operands of the form A + B
   that are not wrapped either in a const or in a memory reference.

   To avoid that error, the usage of the "p" constraint in internal GCC
   instruction templates is supposed to be complemented by the 'a'
   modifier, like in:

     asm volatile ("r1 = %a[jorl]" : : [jorl]"p"(&jorl));

   Internally documented (in GCC's final.cc) as:

     %aN means expect operand N to be a memory address
        (not a memory reference!) and print a reference
        to that address.

   That works because when the modifier 'a' is found, GCC prints an
   "operand address", which is not the same than an "operand".

   But...

2. Even if we used the internal 'a' modifier (we shouldn't) the 'rN =
   rM' instruction really requires a register argument.  In cases
   involving automatics, like in the examples above, we easily end with:

     bar:
        #APP
            r1 = r10-4
        #NO_APP

   In other cases we could conceibly also end with a 64-bit label that
   may overflow the 32-bit immediate operand of `rN = imm32'
   instructions:

        r1 = foo

   All of which is clearly wrong.

clang happens to do "the right thing" in the current usage of __imm_ptr
in the BPF tests, because even with -O2 it seems to "reload" the
fp-relative address of the automatic to a register like in:

  bar:
	r1 = r10
	r1 += -4
	#APP
	r1 = r1
	#NO_APP

Which is what GCC would generate with -O0.  Whether this is by chance
or by design, the compiler shouln't be expected to do that reload
driven by the "p" constraint.

This patch changes the usage of the "p" constraint in the BPF
selftests macros to use the "r" constraint instead.  If a register is
what is required, we should let the compiler know.

Previous discussion in bpf@vger:
https://lore.kernel.org/bpf/87h6p5ebpb.fsf@oracle.com/T/#ef0df83d6975c34dff20bf0dd52e078f5b8ca2767

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240123181309.19853-1-jose.marchesi@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>