powerpc/64: ELFv2 use minimal stack frames in int and switch frame sizes

Adjust the ELFv2 interrupt and switch frames to the minimum C ABI size,
plus pt_regs, plus 16 bytes for the aligned regs marker for the int
frame (and the switch frame needs to match that because it uses the same
regs offset as the int frame).

This saves 80 bytes of kernel stack per interrupt. It's the principle of
getting our accounting right that's more important than the practical
saving.

Signed-off-by: Nicholas Piggin <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: npiggin

arch/powerpc/include/asm/ptrace.h

@@ -120,16 +120,26 @@ struct pt_regs
 
 #define STACK_FRAME_OVERHEAD	112	/* size of minimum stack frame */
 #define STACK_FRAME_LR_SAVE	2	/* Location of LR in stack frame */
+
+#ifdef CONFIG_PPC64_ELF_ABI_V2
+#define STACK_FRAME_MIN_SIZE	32
+#define STACK_USER_INT_FRAME_SIZE	(sizeof(struct pt_regs) + STACK_FRAME_MIN_SIZE + 16)
+#define STACK_INT_FRAME_REGS	(STACK_FRAME_MIN_SIZE + 16)
+#define STACK_INT_FRAME_MARKER	STACK_FRAME_MIN_SIZE
+#define STACK_SWITCH_FRAME_SIZE (sizeof(struct pt_regs) + STACK_FRAME_MIN_SIZE + 16)
+#define STACK_SWITCH_FRAME_REGS	(STACK_FRAME_MIN_SIZE + 16)
+#else
+/*
+ * The ELFv1 ABI specifies 48 bytes plus a minimum 64 byte parameter save
+ * area. This parameter area is not used by calls to C from interrupt entry,
+ * so the second from last one of those is used for the frame marker.
+ */
+#define STACK_FRAME_MIN_SIZE	112
 #define STACK_USER_INT_FRAME_SIZE	(sizeof(struct pt_regs) + STACK_FRAME_OVERHEAD)
 #define STACK_INT_FRAME_REGS	STACK_FRAME_OVERHEAD
 #define STACK_INT_FRAME_MARKER	(STACK_FRAME_OVERHEAD - 16)
 #define STACK_SWITCH_FRAME_SIZE	(sizeof(struct pt_regs) + STACK_FRAME_OVERHEAD)
 #define STACK_SWITCH_FRAME_REGS	STACK_FRAME_OVERHEAD
-
-#ifdef CONFIG_PPC64_ELF_ABI_V2
-#define STACK_FRAME_MIN_SIZE	32
-#else
-#define STACK_FRAME_MIN_SIZE	STACK_FRAME_OVERHEAD
 #endif
 
 /* Size of dummy stack frame allocated when calling signal handler. */