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openrisc: Move FPU state out of pt_regs 

My original, naive, FPU support patch had the FPCSR register stored
during both the *mode switch* and *context switch*.  This is wasteful.

Also, the original patches did not save the FPU state when handling
signals during the system call fast path.

We fix this by moving the FPCSR state to thread_struct in task_struct.
We also introduce new helper functions save_fpu and restore_fpu which
can be used to sync the FPU with thread_struct.  These functions are now
called when needed:

 - Setting up and restoring sigcontext when handling signals
 - Before and after __switch_to during context switches
 - When handling FPU exceptions
 - When reading and writing FPU register sets

In the future we can further optimize this by doing lazy FPU save and
restore.  For example, FPU sync is not needed when switching to and from
kernel threads (x86 does this).  FPU save and restore does not need to
be done two times if we have both rescheduling and signal work to do.
However, since OpenRISC FPU state is a single register, I leave these
optimizations for future consideration.

Signed-off-by: Stafford Horne <shorne@gmail.com>
This commit is contained in:
Stafford Horne 2024-03-30 14:56:39 +00:00
parent 1f33446d0e
commit 4dc70e1aad
8 changed files with 76 additions and 32 deletions
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22
arch/openrisc/include/asm/fpu.h Normal file
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@ -0,0 +1,22 @@
/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __ASM_OPENRISC_FPU_H
#define __ASM_OPENRISC_FPU_H
struct task_struct;
#ifdef CONFIG_FPU
static inline void save_fpu(struct task_struct *task)
{
task->thread.fpcsr = mfspr(SPR_FPCSR);
}
static inline void restore_fpu(struct task_struct *task)
{
mtspr(SPR_FPCSR, task->thread.fpcsr);
}
#else
#define save_fpu(tsk) do { } while (0)
#define restore_fpu(tsk) do { } while (0)
#endif
#endif /* __ASM_OPENRISC_FPU_H */

1
arch/openrisc/include/asm/processor.h
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@ -44,6 +44,7 @@
struct task_struct;
struct thread_struct {
long fpcsr; /* Floating point control status register. */
};
/*

3
arch/openrisc/include/asm/ptrace.h
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@ -59,7 +59,7 @@ struct pt_regs {
* -1 for all other exceptions.
*/
long orig_gpr11; /* For restarting system calls */
long fpcsr; /* Floating point control status register. */
long dummy; /* Cheap alignment fix */
long dummy2; /* Cheap alignment fix */
};
@ -115,6 +115,5 @@ static inline long regs_return_value(struct pt_regs *regs)
#define PT_GPR31 124
#define PT_PC 128
#define PT_ORIG_GPR11 132
#define PT_FPCSR 136
#endif /* __ASM_OPENRISC_PTRACE_H */

15
arch/openrisc/kernel/entry.S
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@ -106,8 +106,6 @@
l.mtspr r0,r3,SPR_EPCR_BASE ;\
l.lwz r3,PT_SR(r1) ;\
l.mtspr r0,r3,SPR_ESR_BASE ;\
l.lwz r3,PT_FPCSR(r1) ;\
l.mtspr r0,r3,SPR_FPCSR ;\
l.lwz r2,PT_GPR2(r1) ;\
l.lwz r3,PT_GPR3(r1) ;\
l.lwz r4,PT_GPR4(r1) ;\
@ -177,8 +175,6 @@ handler: ;\
/* r30 already save */ ;\
l.sw PT_GPR31(r1),r31 ;\
TRACE_IRQS_OFF_ENTRY ;\
l.mfspr r30,r0,SPR_FPCSR ;\
l.sw PT_FPCSR(r1),r30 ;\
/* Store -1 in orig_gpr11 for non-syscall exceptions */ ;\
l.addi r30,r0,-1 ;\
l.sw PT_ORIG_GPR11(r1),r30
@ -219,8 +215,6 @@ handler: ;\
/* Store -1 in orig_gpr11 for non-syscall exceptions */ ;\
l.addi r30,r0,-1 ;\
l.sw PT_ORIG_GPR11(r1),r30 ;\
l.mfspr r30,r0,SPR_FPCSR ;\
l.sw PT_FPCSR(r1),r30 ;\
l.addi r3,r1,0 ;\
/* r4 is exception EA */ ;\
l.addi r5,r0,vector ;\
@ -852,6 +846,7 @@ _syscall_badsys:
EXCEPTION_ENTRY(_fpe_trap_handler)
CLEAR_LWA_FLAG(r3)
/* r4: EA of fault (set by EXCEPTION_HANDLE) */
l.jal do_fpe_trap
l.addi r3,r1,0 /* pt_regs */
@ -1100,10 +1095,6 @@ ENTRY(_switch)
l.sw PT_GPR28(r1),r28
l.sw PT_GPR30(r1),r30
/* Store the old FPU state to new pt_regs */
l.mfspr r29,r0,SPR_FPCSR
l.sw PT_FPCSR(r1),r29
l.addi r11,r10,0 /* Save old 'current' to 'last' return value*/
/* We use thread_info->ksp for storing the address of the above
@ -1126,10 +1117,6 @@ ENTRY(_switch)
l.lwz r29,PT_SP(r1)
l.sw TI_KSP(r10),r29
/* Restore the old value of FPCSR */
l.lwz r29,PT_FPCSR(r1)
l.mtspr r0,r29,SPR_FPCSR
/* ...and restore the registers, except r11 because the return value
* has already been set above.
*/

5
arch/openrisc/kernel/process.c
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@ -36,6 +36,7 @@
#include <linux/reboot.h>
#include <linux/uaccess.h>
#include <asm/fpu.h>
#include <asm/io.h>
#include <asm/processor.h>
#include <asm/spr_defs.h>
@ -244,6 +245,8 @@ struct task_struct *__switch_to(struct task_struct *old,
local_irq_save(flags);
save_fpu(current);
/* current_set is an array of saved current pointers
* (one for each cpu). we need them at user->kernel transition,
* while we save them at kernel->user transition
@ -256,6 +259,8 @@ struct task_struct *__switch_to(struct task_struct *old,
current_thread_info_set[smp_processor_id()] = new_ti;
last = (_switch(old_ti, new_ti))->task;
restore_fpu(current);
local_irq_restore(flags);
return last;

12
arch/openrisc/kernel/ptrace.c
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@ -98,9 +98,7 @@ static int fpregs_get(struct task_struct *target,
const struct user_regset *regset,
struct membuf to)
{
const struct pt_regs *regs = task_pt_regs(target);
return membuf_store(&to, regs->fpcsr);
return membuf_store(&to, target->thread.fpcsr);
}
static int fpregs_set(struct task_struct *target,
@ -108,13 +106,9 @@ static int fpregs_set(struct task_struct *target,
unsigned int pos, unsigned int count,
const void *kbuf, const void __user *ubuf)
{
struct pt_regs *regs = task_pt_regs(target);
int ret;
/* FPCSR */
ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
&regs->fpcsr, 0, 4);
return ret;
return user_regset_copyin(&pos, &count, &kbuf, &ubuf,
&target->thread.fpcsr, 0, 4);
}
#endif

36
arch/openrisc/kernel/signal.c
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@ -23,6 +23,7 @@
#include <linux/stddef.h>
#include <linux/resume_user_mode.h>
#include <asm/fpu.h>
#include <asm/processor.h>
#include <asm/syscall.h>
#include <asm/ucontext.h>
@ -39,6 +40,37 @@ asmlinkage long _sys_rt_sigreturn(struct pt_regs *regs);
asmlinkage int do_work_pending(struct pt_regs *regs, unsigned int thread_flags,
int syscall);
#ifdef CONFIG_FPU
static long restore_fp_state(struct sigcontext __user *sc)
{
long err;
err = __copy_from_user(&current->thread.fpcsr, &sc->fpcsr, sizeof(unsigned long));
if (unlikely(err))
return err;
/* Restore the FPU state */
restore_fpu(current);
return 0;
}
static long save_fp_state(struct sigcontext __user *sc)
{
long err;
/* Sync the user FPU state so we can copy to sigcontext */
save_fpu(current);
err = __copy_to_user(&sc->fpcsr, &current->thread.fpcsr, sizeof(unsigned long));
return err;
}
#else
#define save_fp_state(sc) (0)
#define restore_fp_state(sc) (0)
#endif
static int restore_sigcontext(struct pt_regs *regs,
struct sigcontext __user *sc)
{
@ -55,7 +87,7 @@ static int restore_sigcontext(struct pt_regs *regs,
err |= __copy_from_user(regs, sc->regs.gpr, 32 * sizeof(unsigned long));
err |= __copy_from_user(&regs->pc, &sc->regs.pc, sizeof(unsigned long));
err |= __copy_from_user(&regs->sr, &sc->regs.sr, sizeof(unsigned long));
err |= __copy_from_user(&regs->fpcsr, &sc->fpcsr, sizeof(unsigned long));
err |= restore_fp_state(sc);
/* make sure the SM-bit is cleared so user-mode cannot fool us */
regs->sr &= ~SPR_SR_SM;
@ -118,7 +150,7 @@ static int setup_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc)
err |= __copy_to_user(sc->regs.gpr, regs, 32 * sizeof(unsigned long));
err |= __copy_to_user(&sc->regs.pc, &regs->pc, sizeof(unsigned long));
err |= __copy_to_user(&sc->regs.sr, &regs->sr, sizeof(unsigned long));
err |= __copy_to_user(&sc->fpcsr, &regs->fpcsr, sizeof(unsigned long));
err |= save_fp_state(sc);
return err;
}

14
arch/openrisc/kernel/traps.c
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@ -31,6 +31,7 @@
#include <linux/uaccess.h>
#include <asm/bug.h>
#include <asm/fpu.h>
#include <asm/io.h>
#include <asm/processor.h>
#include <asm/unwinder.h>
@ -84,9 +85,8 @@ void show_registers(struct pt_regs *regs)
in_kernel = 0;
pr_info("CPU #: %d\n"
" PC: %08lx SR: %08lx SP: %08lx FPCSR: %08lx\n",
smp_processor_id(), regs->pc, regs->sr, regs->sp,
regs->fpcsr);
" PC: %08lx SR: %08lx SP: %08lx\n",
smp_processor_id(), regs->pc, regs->sr, regs->sp);
pr_info("GPR00: %08lx GPR01: %08lx GPR02: %08lx GPR03: %08lx\n",
0L, regs->gpr[1], regs->gpr[2], regs->gpr[3]);
pr_info("GPR04: %08lx GPR05: %08lx GPR06: %08lx GPR07: %08lx\n",
@ -183,7 +183,10 @@ asmlinkage void do_fpe_trap(struct pt_regs *regs, unsigned long address)
if (user_mode(regs)) {
int code = FPE_FLTUNK;
#ifdef CONFIG_FPU
unsigned long fpcsr = regs->fpcsr;
unsigned long fpcsr;
save_fpu(current);
fpcsr = current->thread.fpcsr;
if (fpcsr & SPR_FPCSR_IVF)
code = FPE_FLTINV;
@ -197,7 +200,8 @@ asmlinkage void do_fpe_trap(struct pt_regs *regs, unsigned long address)
code = FPE_FLTRES;
/* Clear all flags */
regs->fpcsr &= ~SPR_FPCSR_ALLF;
current->thread.fpcsr &= ~SPR_FPCSR_ALLF;
restore_fpu(current);
#endif
force_sig_fault(SIGFPE, code, (void __user *)regs->pc);
} else {