alpha: lazy FPU switching

	On each context switch we save the FPU registers on stack
of old process and restore FPU registers from the stack of new one.
That allows us to avoid doing that each time we enter/leave the
kernel mode; however, that can get suboptimal in some cases.

	For one thing, we don't need to bother saving anything
for kernel threads.  For another, if between entering and leaving
the kernel a thread gives CPU up more than once, it will do
useless work, saving the same values every time, only to discard
the saved copy as soon as it returns from switch_to().

	Alternative solution:

* move the array we save into from switch_stack to thread_info
* have a (thread-synchronous) flag set when we save them
* have another flag set when they should be restored on return to userland.
* do *NOT* save/restore them in do_switch_stack()/undo_switch_stack().
* restore on the exit to user mode if the restore flag had
been set.  Clear both flags.
* on context switch, entry to fork/clone/vfork, before entry into do_signal()
and on entry into straced syscall save the registers and set the 'saved' flag
unless it had been already set.
* on context switch set the 'restore' flag as well.
* have copy_thread() set both flags for child, so the registers would be
restored once the child returns to userland.
* use the saved data in setup_sigcontext(); have restore_sigcontext() set both flags
and copy from sigframe to save area.
* teach ptrace to look for FPU registers in thread_info instead of
switch_stack.
* teach isolated accesses to FPU registers (rdfpcr, wrfpcr, etc.)
to check the 'saved' flag (under preempt_disable()) and work with the save area
if it's been set; if 'saved' flag is found upon write access, set 'restore' flag
as well.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matt Turner <mattst88@gmail.com>

1 files changed, 73 insertions, 71 deletions

diff --git a/arch/alpha/kernel/entry.S b/arch/alpha/kernel/entry.S
index cc84337afaec..eb51f93a70c8 100644
--- a/arch/alpha/kernel/entry.S
+++ b/arch/alpha/kernel/entry.S
@@ -17,7 +17,7 @@
 
 /* Stack offsets.  */
 #define SP_OFF			184
-#define SWITCH_STACK_SIZE	320
+#define SWITCH_STACK_SIZE	64
 
 .macro	CFI_START_OSF_FRAME	func
 	.align	4
@@ -159,7 +159,6 @@
 	.cfi_rel_offset	$13, 32
 	.cfi_rel_offset	$14, 40
 	.cfi_rel_offset	$15, 48
-	/* We don't really care about the FP registers for debugging.  */
 .endm
 
 .macro	UNDO_SWITCH_STACK
@@ -498,6 +497,10 @@ ret_to_user:
 	and	$17, _TIF_WORK_MASK, $2
 	bne	$2, work_pending
 restore_all:
+	ldl	$2, TI_STATUS($8)
+	and	$2, TS_SAVED_FP | TS_RESTORE_FP, $3
+	bne	$3, restore_fpu
+restore_other:
 	.cfi_remember_state
 	RESTORE_ALL
 	call_pal PAL_rti
@@ -506,7 +509,7 @@ ret_to_kernel:
 	.cfi_restore_state
 	lda	$16, 7
 	call_pal PAL_swpipl
-	br restore_all
+	br restore_other
 
 	.align 3
 $syscall_error:
@@ -570,6 +573,14 @@ $work_notifysig:
 	.type	strace, @function
 strace:
 	/* set up signal stack, call syscall_trace */
+	// NB: if anyone adds preemption, this block will need to be protected
+	ldl	$1, TI_STATUS($8)
+	and	$1, TS_SAVED_FP, $3
+	or	$1, TS_SAVED_FP, $2
+	bne	$3, 1f
+	stl	$2, TI_STATUS($8)
+	bsr	$26, __save_fpu
+1:
 	DO_SWITCH_STACK
 	jsr	$26, syscall_trace_enter /* returns the syscall number */
 	UNDO_SWITCH_STACK
@@ -649,40 +660,6 @@ do_switch_stack:
 	stq	$14, 40($sp)
 	stq	$15, 48($sp)
 	stq	$26, 56($sp)
-	stt	$f0, 64($sp)
-	stt	$f1, 72($sp)
-	stt	$f2, 80($sp)
-	stt	$f3, 88($sp)
-	stt	$f4, 96($sp)
-	stt	$f5, 104($sp)
-	stt	$f6, 112($sp)
-	stt	$f7, 120($sp)
-	stt	$f8, 128($sp)
-	stt	$f9, 136($sp)
-	stt	$f10, 144($sp)
-	stt	$f11, 152($sp)
-	stt	$f12, 160($sp)
-	stt	$f13, 168($sp)
-	stt	$f14, 176($sp)
-	stt	$f15, 184($sp)
-	stt	$f16, 192($sp)
-	stt	$f17, 200($sp)
-	stt	$f18, 208($sp)
-	stt	$f19, 216($sp)
-	stt	$f20, 224($sp)
-	stt	$f21, 232($sp)
-	stt	$f22, 240($sp)
-	stt	$f23, 248($sp)
-	stt	$f24, 256($sp)
-	stt	$f25, 264($sp)
-	stt	$f26, 272($sp)
-	stt	$f27, 280($sp)
-	mf_fpcr	$f0		# get fpcr
-	stt	$f28, 288($sp)
-	stt	$f29, 296($sp)
-	stt	$f30, 304($sp)
-	stt	$f0, 312($sp)	# save fpcr in slot of $f31
-	ldt	$f0, 64($sp)	# dont let "do_switch_stack" change fp state.
 	ret	$31, ($1), 1
 	.cfi_endproc
 	.size	do_switch_stack, .-do_switch_stack
@@ -701,54 +678,71 @@ undo_switch_stack:
 	ldq	$14, 40($sp)
 	ldq	$15, 48($sp)
 	ldq	$26, 56($sp)
-	ldt	$f30, 312($sp)	# get saved fpcr
-	ldt	$f0, 64($sp)
-	ldt	$f1, 72($sp)
-	ldt	$f2, 80($sp)
-	ldt	$f3, 88($sp)
-	mt_fpcr	$f30		# install saved fpcr
-	ldt	$f4, 96($sp)
-	ldt	$f5, 104($sp)
-	ldt	$f6, 112($sp)
-	ldt	$f7, 120($sp)
-	ldt	$f8, 128($sp)
-	ldt	$f9, 136($sp)
-	ldt	$f10, 144($sp)
-	ldt	$f11, 152($sp)
-	ldt	$f12, 160($sp)
-	ldt	$f13, 168($sp)
-	ldt	$f14, 176($sp)
-	ldt	$f15, 184($sp)
-	ldt	$f16, 192($sp)
-	ldt	$f17, 200($sp)
-	ldt	$f18, 208($sp)
-	ldt	$f19, 216($sp)
-	ldt	$f20, 224($sp)
-	ldt	$f21, 232($sp)
-	ldt	$f22, 240($sp)
-	ldt	$f23, 248($sp)
-	ldt	$f24, 256($sp)
-	ldt	$f25, 264($sp)
-	ldt	$f26, 272($sp)
-	ldt	$f27, 280($sp)
-	ldt	$f28, 288($sp)
-	ldt	$f29, 296($sp)
-	ldt	$f30, 304($sp)
 	lda	$sp, SWITCH_STACK_SIZE($sp)
 	ret	$31, ($1), 1
 	.cfi_endproc
 	.size	undo_switch_stack, .-undo_switch_stack
+
+#define FR(n) n * 8 + TI_FP($8)
+	.align	4
+	.globl	__save_fpu
+	.type	__save_fpu, @function
+__save_fpu:
+#define V(n) stt	$f##n, FR(n)
+	V( 0); V( 1); V( 2); V( 3)
+	V( 4); V( 5); V( 6); V( 7)
+	V( 8); V( 9); V(10); V(11)
+	V(12); V(13); V(14); V(15)
+	V(16); V(17); V(18); V(19)
+	V(20); V(21); V(22); V(23)
+	V(24); V(25); V(26); V(27)
+	mf_fpcr	$f0		# get fpcr
+	V(28); V(29); V(30)
+	stt	$f0, FR(31)	# save fpcr in slot of $f31
+	ldt	$f0, FR(0)	# don't let "__save_fpu" change fp state.
+	ret
+#undef V
+	.size	__save_fpu, .-__save_fpu
+
+	.align	4
+restore_fpu:
+	and	$3, TS_RESTORE_FP, $3
+	bic	$2, TS_SAVED_FP | TS_RESTORE_FP, $2
+	beq	$3, 1f
+#define V(n) ldt	$f##n, FR(n)
+	ldt	$f30, FR(31)	# get saved fpcr
+	V( 0); V( 1); V( 2); V( 3)
+	mt_fpcr	$f30		# install saved fpcr
+	V( 4); V( 5); V( 6); V( 7)
+	V( 8); V( 9); V(10); V(11)
+	V(12); V(13); V(14); V(15)
+	V(16); V(17); V(18); V(19)
+	V(20); V(21); V(22); V(23)
+	V(24); V(25); V(26); V(27)
+	V(28); V(29); V(30)
+1:	stl $2, TI_STATUS($8)
+	br restore_other
+#undef V
+
 
 /*
  * The meat of the context switch code.
  */
-
 	.align	4
 	.globl	alpha_switch_to
 	.type	alpha_switch_to, @function
 	.cfi_startproc
 alpha_switch_to:
 	DO_SWITCH_STACK
+	ldl	$1, TI_STATUS($8)
+	and	$1, TS_RESTORE_FP, $3
+	bne	$3, 1f
+	or	$1, TS_RESTORE_FP | TS_SAVED_FP, $2
+	and	$1, TS_SAVED_FP, $3
+	stl	$2, TI_STATUS($8)
+	bne	$3, 1f
+	bsr	$26, __save_fpu
+1:
 	call_pal PAL_swpctx
 	lda	$8, 0x3fff
 	UNDO_SWITCH_STACK
@@ -799,6 +793,14 @@ ret_from_kernel_thread:
 alpha_\name:
 	.prologue 0
 	bsr	$1, do_switch_stack
+	// NB: if anyone adds preemption, this block will need to be protected
+	ldl	$1, TI_STATUS($8)
+	and	$1, TS_SAVED_FP, $3
+	or	$1, TS_SAVED_FP, $2
+	bne	$3, 1f
+	stl	$2, TI_STATUS($8)
+	bsr	$26, __save_fpu
+1:
 	jsr	$26, sys_\name
 	ldq	$26, 56($sp)
 	lda	$sp, SWITCH_STACK_SIZE($sp)