[PATCH] ARM: fix alignement of __bug_table section entries

Russell King - ARM Linux [off-list ref] writes:

I've been wondering whether we can teach GCC that set_domain modifies
the value that get_domain returns, rather than throwing a volatile
onto the asm in get_domain.  The issue with a volatile there is that
even if the result is unused, but the code is reachable, gcc still has
to output the code to read the register.

We might be able to get away with a memory clobber on the set_domain,
and fake a memory read in get_domain, eg, by passing
	"m" (current_thread_info()->cpu_domain))
to the get_domain asm.

Ok, let's say we do it that way.

I have some concerns about it:
  (a) I see an inbalance, as set_domain() doesn't actually modify
      current_thread_info()->cpu_domain. I don't see how it will protect use
      from this scenario :
        - get_domain()
        - set_domain()
        - set_domain()

  (b) domain.h is included from thread_info.h, not the other way around
      => current_thread_info() is not accessible from domain.h
      => that would require a bit of moving things around, as thread_info
         structure description should be available for example.
      This is currently my biggest problem with this approach.

  (c) I was also wondering if a case like this could happen :
     - a function foo() does a get_domain()
       => an exception/irq whatever happens and modifies the DACR
     - foo() continues a makes a modify_domain()
       => and here the modify_domain() uses the old DACR value
      Or said differently, I wonder if there is a case of 2 get_domain() calls
      in a row with a DACR modification in between. I

What about something such as [1], without a memory clobber, but a "fake" memory
variable link ?

Cheers.

--
Robert

[1] get_domain() / set_domain() link

diff --git a/arch/arm/include/asm/domain.h b/arch/arm/include/asm/domain.h
index e878129f2fee..fc1d9c43aa08 100644
--- a/arch/arm/include/asm/domain.h
+++ b/arch/arm/include/asm/domain.h

@@ -83,13 +83,17 @@
 
 #ifndef __ASSEMBLY__
 
+static int domain_barrier;
+/*
+ * how to get the current stack pointer in C
+ */
 static inline unsigned int get_domain(void)
 {
        unsigned int domain;
 
        asm(
        "mrc    p15, 0, %0, c3, c0      @ get domain"
-        : "=r" (domain));
+        : "=r" (domain), "=m" (domain_barrier));
 
        return domain;
 }

@@ -97,8 +101,8 @@ static inline unsigned int get_domain(void)
 static inline void set_domain(unsigned val)
 {
        asm volatile(
-       "mcr    p15, 0, %0, c3, c0      @ set domain"
-         : : "r" (val));
+       "mcr    p15, 0, %1, c3, c0      @ set domain"
+       : "=m" (domain_barrier) : "r" (val));
        isb();
 }