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alpha: lazy FPU switching

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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>
This commit is contained in:
Al Viro 2022-09-02 02:50:12 +01:00 committed by Matt Turner
parent a7acb188e8
commit 0509666660
9 changed files with 192 additions and 119 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

61
arch/alpha/include/asm/fpu.h
View File

@ -15,21 +15,27 @@ rdfpcr(void)
{
unsigned long tmp, ret;
preempt_disable();
if (current_thread_info()->status & TS_SAVED_FP) {
ret = current_thread_info()->fp[31];
} else {
#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
__asm__ __volatile__ (
"ftoit $f0,%0\n\t"
"mf_fpcr $f0\n\t"
"ftoit $f0,%1\n\t"
"itoft %0,$f0"
: "=r"(tmp), "=r"(ret));
__asm__ __volatile__ (
"ftoit $f0,%0\n\t"
"mf_fpcr $f0\n\t"
"ftoit $f0,%1\n\t"
"itoft %0,$f0"
: "=r"(tmp), "=r"(ret));
#else
__asm__ __volatile__ (
"stt $f0,%0\n\t"
"mf_fpcr $f0\n\t"
"stt $f0,%1\n\t"
"ldt $f0,%0"
: "=m"(tmp), "=m"(ret));
__asm__ __volatile__ (
"stt $f0,%0\n\t"
"mf_fpcr $f0\n\t"
"stt $f0,%1\n\t"
"ldt $f0,%0"
: "=m"(tmp), "=m"(ret));
#endif
}
preempt_enable();
return ret;
}
@ -39,21 +45,28 @@ wrfpcr(unsigned long val)
{
unsigned long tmp;
preempt_disable();
if (current_thread_info()->status & TS_SAVED_FP) {
current_thread_info()->status |= TS_RESTORE_FP;
current_thread_info()->fp[31] = val;
} else {
#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
__asm__ __volatile__ (
"ftoit $f0,%0\n\t"
"itoft %1,$f0\n\t"
"mt_fpcr $f0\n\t"
"itoft %0,$f0"
: "=&r"(tmp) : "r"(val));
__asm__ __volatile__ (
"ftoit $f0,%0\n\t"
"itoft %1,$f0\n\t"
"mt_fpcr $f0\n\t"
"itoft %0,$f0"
: "=&r"(tmp) : "r"(val));
#else
__asm__ __volatile__ (
"stt $f0,%0\n\t"
"ldt $f0,%1\n\t"
"mt_fpcr $f0\n\t"
"ldt $f0,%0"
: "=m"(tmp) : "m"(val));
__asm__ __volatile__ (
"stt $f0,%0\n\t"
"ldt $f0,%1\n\t"
"mt_fpcr $f0\n\t"
"ldt $f0,%0"
: "=m"(tmp) : "m"(val));
#endif
}
preempt_enable();
}
static inline unsigned long

16
arch/alpha/include/asm/thread_info.h
View File

@ -26,6 +26,7 @@ struct thread_info {
int bpt_nsaved;
unsigned long bpt_addr[2]; /* breakpoint handling */
unsigned int bpt_insn[2];
unsigned long fp[32];
};
/*
@ -83,6 +84,9 @@ register unsigned long *current_stack_pointer __asm__ ("$30");
#define TS_UAC_NOFIX 0x0002 /* ! flags as they match */
#define TS_UAC_SIGBUS 0x0004 /* ! userspace part of 'osf_sysinfo' */
#define TS_SAVED_FP 0x0008
#define TS_RESTORE_FP 0x0010
#define SET_UNALIGN_CTL(task,value) ({ \
__u32 status = task_thread_info(task)->status & ~UAC_BITMASK; \
if (value & PR_UNALIGN_NOPRINT) \
@ -106,5 +110,17 @@ register unsigned long *current_stack_pointer __asm__ ("$30");
put_user(res, (int __user *)(value)); \
})
#ifndef __ASSEMBLY__
extern void __save_fpu(void);
static inline void save_fpu(void)
{
if (!(current_thread_info()->status & TS_SAVED_FP)) {
current_thread_info()->status |= TS_SAVED_FP;
__save_fpu();
}
}
#endif
#endif /* __KERNEL__ */
#endif /* _ALPHA_THREAD_INFO_H */

2
arch/alpha/include/uapi/asm/ptrace.h
View File

@ -64,7 +64,9 @@ struct switch_stack {
unsigned long r14;
unsigned long r15;
unsigned long r26;
#ifndef __KERNEL__
unsigned long fp[32]; /* fp[31] is fpcr */
#endif
};

2
arch/alpha/kernel/asm-offsets.c
View File

@ -17,6 +17,8 @@ void foo(void)
DEFINE(TI_TASK, offsetof(struct thread_info, task));
DEFINE(TI_FLAGS, offsetof(struct thread_info, flags));
DEFINE(TI_CPU, offsetof(struct thread_info, cpu));
DEFINE(TI_FP, offsetof(struct thread_info, fp));
DEFINE(TI_STATUS, offsetof(struct thread_info, status));
BLANK();
DEFINE(TASK_BLOCKED, offsetof(struct task_struct, blocked));

144
arch/alpha/kernel/entry.S
View File

@ -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)

5
arch/alpha/kernel/process.c
View File

@ -244,6 +244,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
childstack = ((struct switch_stack *) childregs) - 1;
childti->pcb.ksp = (unsigned long) childstack;
childti->pcb.flags = 1; /* set FEN, clear everything else */
childti->status |= TS_SAVED_FP | TS_RESTORE_FP;
if (unlikely(args->fn)) {
/* kernel thread */
@ -253,6 +254,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
childstack->r9 = (unsigned long) args->fn;
childstack->r10 = (unsigned long) args->fn_arg;
childregs->hae = alpha_mv.hae_cache;
memset(childti->fp, '\0', sizeof(childti->fp));
childti->pcb.usp = 0;
return 0;
}
@ -335,8 +337,7 @@ EXPORT_SYMBOL(dump_elf_task);
int elf_core_copy_task_fpregs(struct task_struct *t, elf_fpregset_t *fpu)
{
struct switch_stack *sw = (struct switch_stack *)task_pt_regs(t) - 1;
memcpy(fpu, sw->fp, 32 * 8);
memcpy(fpu, task_thread_info(t)->fp, 32 * 8);
return 1;
}

18
arch/alpha/kernel/ptrace.c
View File

@ -78,6 +78,8 @@ enum {
(PAGE_SIZE*2 - sizeof(struct pt_regs) - sizeof(struct switch_stack) \
+ offsetof(struct switch_stack, reg))
#define FP_REG(reg) (offsetof(struct thread_info, reg))
static int regoff[] = {
PT_REG( r0), PT_REG( r1), PT_REG( r2), PT_REG( r3),
PT_REG( r4), PT_REG( r5), PT_REG( r6), PT_REG( r7),
@ -87,14 +89,14 @@ static int regoff[] = {
PT_REG( r20), PT_REG( r21), PT_REG( r22), PT_REG( r23),
PT_REG( r24), PT_REG( r25), PT_REG( r26), PT_REG( r27),
PT_REG( r28), PT_REG( gp), -1, -1,
SW_REG(fp[ 0]), SW_REG(fp[ 1]), SW_REG(fp[ 2]), SW_REG(fp[ 3]),
SW_REG(fp[ 4]), SW_REG(fp[ 5]), SW_REG(fp[ 6]), SW_REG(fp[ 7]),
SW_REG(fp[ 8]), SW_REG(fp[ 9]), SW_REG(fp[10]), SW_REG(fp[11]),
SW_REG(fp[12]), SW_REG(fp[13]), SW_REG(fp[14]), SW_REG(fp[15]),
SW_REG(fp[16]), SW_REG(fp[17]), SW_REG(fp[18]), SW_REG(fp[19]),
SW_REG(fp[20]), SW_REG(fp[21]), SW_REG(fp[22]), SW_REG(fp[23]),
SW_REG(fp[24]), SW_REG(fp[25]), SW_REG(fp[26]), SW_REG(fp[27]),
SW_REG(fp[28]), SW_REG(fp[29]), SW_REG(fp[30]), SW_REG(fp[31]),
FP_REG(fp[ 0]), FP_REG(fp[ 1]), FP_REG(fp[ 2]), FP_REG(fp[ 3]),
FP_REG(fp[ 4]), FP_REG(fp[ 5]), FP_REG(fp[ 6]), FP_REG(fp[ 7]),
FP_REG(fp[ 8]), FP_REG(fp[ 9]), FP_REG(fp[10]), FP_REG(fp[11]),
FP_REG(fp[12]), FP_REG(fp[13]), FP_REG(fp[14]), FP_REG(fp[15]),
FP_REG(fp[16]), FP_REG(fp[17]), FP_REG(fp[18]), FP_REG(fp[19]),
FP_REG(fp[20]), FP_REG(fp[21]), FP_REG(fp[22]), FP_REG(fp[23]),
FP_REG(fp[24]), FP_REG(fp[25]), FP_REG(fp[26]), FP_REG(fp[27]),
FP_REG(fp[28]), FP_REG(fp[29]), FP_REG(fp[30]), FP_REG(fp[31]),
PT_REG( pc)
};

20
arch/alpha/kernel/signal.c
View File

@ -150,9 +150,10 @@ restore_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs)
{
unsigned long usp;
struct switch_stack *sw = (struct switch_stack *)regs - 1;
long i, err = __get_user(regs->pc, &sc->sc_pc);
long err = __get_user(regs->pc, &sc->sc_pc);
current->restart_block.fn = do_no_restart_syscall;
current_thread_info()->status |= TS_SAVED_FP | TS_RESTORE_FP;
sw->r26 = (unsigned long) ret_from_sys_call;
@ -189,9 +190,9 @@ restore_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs)
err |= __get_user(usp, sc->sc_regs+30);
wrusp(usp);
for (i = 0; i < 31; i++)
err |= __get_user(sw->fp[i], sc->sc_fpregs+i);
err |= __get_user(sw->fp[31], &sc->sc_fpcr);
err |= __copy_from_user(current_thread_info()->fp,
sc->sc_fpregs, 31 * 8);
err |= __get_user(current_thread_info()->fp[31], &sc->sc_fpcr);
return err;
}
@ -272,7 +273,7 @@ setup_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs,
unsigned long mask, unsigned long sp)
{
struct switch_stack *sw = (struct switch_stack *)regs - 1;
long i, err = 0;
long err = 0;
err |= __put_user(on_sig_stack((unsigned long)sc), &sc->sc_onstack);
err |= __put_user(mask, &sc->sc_mask);
@ -312,10 +313,10 @@ setup_sigcontext(struct sigcontext __user *sc, struct pt_regs *regs,
err |= __put_user(sp, sc->sc_regs+30);
err |= __put_user(0, sc->sc_regs+31);
for (i = 0; i < 31; i++)
err |= __put_user(sw->fp[i], sc->sc_fpregs+i);
err |= __copy_to_user(sc->sc_fpregs,
current_thread_info()->fp, 31 * 8);
err |= __put_user(0, sc->sc_fpregs+31);
err |= __put_user(sw->fp[31], &sc->sc_fpcr);
err |= __put_user(current_thread_info()->fp[31], &sc->sc_fpcr);
err |= __put_user(regs->trap_a0, &sc->sc_traparg_a0);
err |= __put_user(regs->trap_a1, &sc->sc_traparg_a1);
@ -528,6 +529,9 @@ do_work_pending(struct pt_regs *regs, unsigned long thread_flags,
} else {
local_irq_enable();
if (thread_flags & (_TIF_SIGPENDING|_TIF_NOTIFY_SIGNAL)) {
preempt_disable();
save_fpu();
preempt_enable();
do_signal(regs, r0, r19);
r0 = 0;
} else {

43
arch/alpha/lib/fpreg.c
View File

@ -7,6 +7,8 @@
#include <linux/compiler.h>
#include <linux/export.h>
#include <linux/preempt.h>
#include <asm/thread_info.h>
#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
#define STT(reg,val) asm volatile ("ftoit $f"#reg",%0" : "=r"(val));
@ -19,7 +21,12 @@ alpha_read_fp_reg (unsigned long reg)
{
unsigned long val;
switch (reg) {
if (unlikely(reg >= 32))
return 0;
preempt_enable();
if (current_thread_info()->status & TS_SAVED_FP)
val = current_thread_info()->fp[reg];
else switch (reg) {
case 0: STT( 0, val); break;
case 1: STT( 1, val); break;
case 2: STT( 2, val); break;
@ -52,8 +59,8 @@ alpha_read_fp_reg (unsigned long reg)
case 29: STT(29, val); break;
case 30: STT(30, val); break;
case 31: STT(31, val); break;
default: return 0;
}
preempt_enable();
return val;
}
EXPORT_SYMBOL(alpha_read_fp_reg);
@ -67,7 +74,14 @@ EXPORT_SYMBOL(alpha_read_fp_reg);
void
alpha_write_fp_reg (unsigned long reg, unsigned long val)
{
switch (reg) {
if (unlikely(reg >= 32))
return;
preempt_disable();
if (current_thread_info()->status & TS_SAVED_FP) {
current_thread_info()->status |= TS_RESTORE_FP;
current_thread_info()->fp[reg] = val;
} else switch (reg) {
case 0: LDT( 0, val); break;
case 1: LDT( 1, val); break;
case 2: LDT( 2, val); break;
@ -101,6 +115,7 @@ alpha_write_fp_reg (unsigned long reg, unsigned long val)
case 30: LDT(30, val); break;
case 31: LDT(31, val); break;
}
preempt_enable();
}
EXPORT_SYMBOL(alpha_write_fp_reg);
@ -115,7 +130,14 @@ alpha_read_fp_reg_s (unsigned long reg)
{
unsigned long val;
switch (reg) {
if (unlikely(reg >= 32))
return 0;
preempt_enable();
if (current_thread_info()->status & TS_SAVED_FP) {
LDT(0, current_thread_info()->fp[reg]);
STS(0, val);
} else switch (reg) {
case 0: STS( 0, val); break;
case 1: STS( 1, val); break;
case 2: STS( 2, val); break;
@ -148,8 +170,8 @@ alpha_read_fp_reg_s (unsigned long reg)
case 29: STS(29, val); break;
case 30: STS(30, val); break;
case 31: STS(31, val); break;
default: return 0;
}
preempt_enable();
return val;
}
EXPORT_SYMBOL(alpha_read_fp_reg_s);
@ -163,7 +185,15 @@ EXPORT_SYMBOL(alpha_read_fp_reg_s);
void
alpha_write_fp_reg_s (unsigned long reg, unsigned long val)
{
switch (reg) {
if (unlikely(reg >= 32))
return;
preempt_disable();
if (current_thread_info()->status & TS_SAVED_FP) {
current_thread_info()->status |= TS_RESTORE_FP;
LDS(0, val);
STT(0, current_thread_info()->fp[reg]);
} else switch (reg) {
case 0: LDS( 0, val); break;
case 1: LDS( 1, val); break;
case 2: LDS( 2, val); break;
@ -197,5 +227,6 @@ alpha_write_fp_reg_s (unsigned long reg, unsigned long val)
case 30: LDS(30, val); break;
case 31: LDS(31, val); break;
}
preempt_enable();
}
EXPORT_SYMBOL(alpha_write_fp_reg_s);