Unverified Commit cc97ab23 authored by Paul Burton's avatar Paul Burton
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MIPS: Simplify FP context initialization 



MIPS has up until now had 3 different ways for a task's floating point
context to be initialized:

  - If the task's first use of FP involves it gaining ownership of an
    FPU then _init_fpu() is used to initialize the FPU's registers such
    that they all contain ~0, and the FPU registers will be stored to
    struct thread_info later (eg. when context switching).

  - If the task first uses FP on a CPU without an associated FPU then
    fpu_emulator_init_fpu() initializes the task's floating point
    register state in struct thread_info such that all floating point
    register contain the bit pattern 0x7ff800007ff80000, different to
    the _init_fpu() behaviour.

  - If a task's floating point context is first accessed via ptrace then
    init_fp_ctx() initializes the floating point register state in
    struct thread_info to ~0, giving equivalent state to _init_fpu().

The _init_fpu() path has 2 separate implementations - one for r2k/r3k
style systems & one for r4k style systems. The _init_fpu() path also
requires that we be careful to clear & restore the value of the
Config5.FRE bit on modern systems in order to avoid inadvertently
triggering floating point exceptions.

None of this code is in a performance critical hot path - it runs only
the first time a task uses floating point. As such it doesn't seem to
warrant the complications of maintaining the _init_fpu() path.

Remove _init_fpu() & fpu_emulator_init_fpu(), instead using
init_fp_ctx() consistently to initialize floating point register state
in struct thread_info. Upon a task's first use of floating point this
will typically mean that we initialize state in memory & then load it
into FPU registers using _restore_fp() just as we would on a context
switch. For other paths such as __compute_return_epc_for_insn() or
mipsr2_decoder() this results in a significant simplification of the
work to be done.

Signed-off-by: default avatarPaul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21002/
Cc: linux-mips@linux-mips.org
parent b1013f1e

arch/mips/include/asm/fpu.h

+25 −32
Original line number Diff line number Diff line
@@ -33,7 +33,6 @@ 
struct sigcontext;

struct sigcontext32;



extern void _init_fpu(unsigned int);

extern void _save_fp(struct task_struct *);

extern void _restore_fp(struct task_struct *);
@@ -198,42 +197,36 @@ static inline void lose_fpu(int save) 
	preempt_enable();

}



static inline int init_fpu(void)

/**

 * init_fp_ctx() - Initialize task FP context

 * @target: The task whose FP context should be initialized.

 *

 * Initializes the FP context of the target task to sane default values if that

 * target task does not already have valid FP context. Once the context has

 * been initialized, the task will be marked as having used FP & thus having

 * valid FP context.

 *

 * Returns: true if context is initialized, else false.

 */

static inline bool init_fp_ctx(struct task_struct *target)

{

	unsigned int fcr31 = current->thread.fpu.fcr31;

	int ret = 0;



	if (cpu_has_fpu) {

		unsigned int config5;



		ret = __own_fpu();

		if (ret)

			return ret;

	/* If FP has been used then the target already has context */

	if (tsk_used_math(target))

		return false;



		if (!cpu_has_fre) {

			_init_fpu(fcr31);

	/* Begin with data registers set to all 1s... */

	memset(&target->thread.fpu.fpr, ~0, sizeof(target->thread.fpu.fpr));



			return 0;

		}

	/* FCSR has been preset by `mips_set_personality_nan'.  */



	/*

		 * Ensure FRE is clear whilst running _init_fpu, since

		 * single precision FP instructions are used. If FRE

		 * was set then we'll just end up initialising all 32

		 * 64b registers.

	 * Record that the target has "used" math, such that the context

	 * just initialised, and any modifications made by the caller,

	 * aren't discarded.

	 */

		config5 = clear_c0_config5(MIPS_CONF5_FRE);

		enable_fpu_hazard();

	set_stopped_child_used_math(target);



		_init_fpu(fcr31);



		/* Restore FRE */

		write_c0_config5(config5);

		enable_fpu_hazard();

	} else

		fpu_emulator_init_fpu();



	return ret;

	return true;

}



static inline void save_fp(struct task_struct *tsk)

arch/mips/include/asm/fpu_emulator.h

+0 −11
Original line number Diff line number Diff line
@@ -188,17 +188,6 @@ int isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn, 
int mm_isBranchInstr(struct pt_regs *regs, struct mm_decoded_insn dec_insn,

		     unsigned long *contpc);



#define SIGNALLING_NAN 0x7ff800007ff80000LL



static inline void fpu_emulator_init_fpu(void)

{

	struct task_struct *t = current;

	int i;



	for (i = 0; i < 32; i++)

		set_fpr64(&t->thread.fpu.fpr[i], 0, SIGNALLING_NAN);

}



/*

 * Mask the FCSR Cause bits according to the Enable bits, observing

 * that Unimplemented is always enabled.

arch/mips/kernel/branch.c

+2 −10
Original line number Diff line number Diff line
@@ -674,16 +674,8 @@ int __compute_return_epc_for_insn(struct pt_regs *regs, 
		if (cpu_has_mips_r6 &&

		    ((insn.i_format.rs == bc1eqz_op) ||

		     (insn.i_format.rs == bc1nez_op))) {

			if (!used_math()) { /* First time FPU user */

				ret = init_fpu();

				if (ret && NO_R6EMU) {

					ret = -ret;

					break;

				}

				ret = 0;

				set_used_math();

			}

			lose_fpu(1);    /* Save FPU state for the emulator. */

			if (!init_fp_ctx(current))

				lose_fpu(1);

			reg = insn.i_format.rt;

			bit = get_fpr32(&current->thread.fpu.fpr[reg], 0) & 0x1;

			if (insn.i_format.rs == bc1eqz_op)

arch/mips/kernel/mips-r2-to-r6-emul.c

+3 −7
Original line number Diff line number Diff line
@@ -1174,13 +1174,9 @@ repeat: 
fpu_emul:

		regs->regs[31] = r31;

		regs->cp0_epc = epc;

		if (!used_math()) {     /* First time FPU user.  */

			preempt_disable();

			err = init_fpu();

			preempt_enable();

			set_used_math();

		}

		lose_fpu(1);    /* Save FPU state for the emulator. */



		if (!init_fp_ctx(current))

			lose_fpu(1);



		err = fpu_emulator_cop1Handler(regs, &current->thread.fpu, 0,

					       &fault_addr);

arch/mips/kernel/ptrace.c

+0 −19
Original line number Diff line number Diff line
@@ -50,25 +50,6 @@ 
#define CREATE_TRACE_POINTS

#include <trace/events/syscalls.h>



static void init_fp_ctx(struct task_struct *target)

{

	/* If FP has been used then the target already has context */

	if (tsk_used_math(target))

		return;



	/* Begin with data registers set to all 1s... */

	memset(&target->thread.fpu.fpr, ~0, sizeof(target->thread.fpu.fpr));



	/* FCSR has been preset by `mips_set_personality_nan'.  */



	/*

	 * Record that the target has "used" math, such that the context

	 * just initialised, and any modifications made by the caller,

	 * aren't discarded.

	 */

	set_stopped_child_used_math(target);

}



/*

 * Called by kernel/ptrace.c when detaching..

 *

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