In message [ref] you wrote
:

On Jun 18, 2008, at 11:35 PM, Michael Neuling wrote:

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In message <5AEB0769-1394-4924-803D- 
C40CAF685519@kernel.crashing.org> you wrote
:

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Index: linux-2.6-ozlabs/include/asm-powerpc/processor.h
===================================================================

--- linux-2.6-ozlabs.orig/include/asm-powerpc/processor.h
+++ linux-2.6-ozlabs/include/asm-powerpc/processor.h

@@ -78,6 +78,7 @@ extern long kernel_thread(int (*fn)(void

/* Lazy FPU handling on uni-processor */
extern struct task_struct *last_task_used_math;
extern struct task_struct *last_task_used_altivec;
+extern struct task_struct *last_task_used_vsx;
extern struct task_struct *last_task_used_spe;

#ifdef CONFIG_PPC32

@@ -136,8 +137,13 @@ typedef struct {

	unsigned long seg;
} mm_segment_t;

+#ifdef CONFIG_VSX
+#define TS_FPR(i) fpvsr.fp[i].fpr
+#define TS_FPRSTART fpvsr.fp
+#else
#define TS_FPR(i) fpr[i]
#define TS_FPRSTART fpr
+#endif

struct thread_struct {
	unsigned long	ksp;		/* Kernel stack pointer */

@@ -155,8 +161,19 @@ struct thread_struct {

	unsigned long	dbcr0;		/* debug control register values */
	unsigned long	dbcr1;
#endif
+#ifdef CONFIG_VSX
+	/* First 32 VSX registers (overlap with fpr[32]) */
+	union {
+		struct {
+			double fpr;
+			double vsrlow;
+		} fp[32];
+		vector128	vsr[32];

how about:

	union {
		struct {
			double fp;
			double vsrlow;
		} fpr;
		vector128 v;
	} fpvsr[32];

Arrh, yep, makes more sense to put the array definition outside the
union.  I'll change.

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+	} fpvsr __attribute__((aligned(16)));

Do we really need a union here?  what would happen if you just  
changed
the type of fpr[32] from double to vector if #CONFIG_VSX?

I really dont like the union and think we can just make the storage
look opaque which is the key.  I doubt we every really care about
using fpr[] as a double in the kernel.

I did something similar to this for the first cut of this patch, but  
it
made the code accessing this structure much less readable.

really, what code is that?

Any code that has to read/write the top or bottom 64 bits _only_ of the
128 bit vector.

The signals code is a good example where, for backwards compatibility,
we need to read/write the old 64 bit FP regs, from the 128 bit value in
the struct.

Similarly, the way we've extended the signals interface for VSX, you
need to read/write out the bottom 64 bits (vsrlow) of a 128 bit value.

eg. the simple:
     current->thread.fpvsr.fp[i].vsrlow = buf[i]
would turn into some abomination/macro.

Mikey