linux/arch/arm/nwfpe/entry.S

/* SPDX-License-Identifier: GPL-2.0-or-later */
/*
    NetWinder Floating Point Emulator
    (c) Rebel.COM, 1998
    (c) 1998, 1999 Philip Blundell

    Direct questions, comments to Scott Bambrough <scottb@netwinder.org>

*/
#include <linux/linkage.h>
#include <asm/assembler.h>
#include <asm/opcodes.h>

/* This is the kernel's entry point into the floating point emulator.
It is called from the kernel with code similar to this:

	sub	r4, r5, #4
	ldrt	r0, [r4]			@ r0  = instruction
	adrsvc	al, r9, ret_from_exception	@ r9  = normal FP return
	adrsvc	al, lr, fpundefinstr		@ lr  = undefined instr return

	get_current_task r10
	mov	r8, #1
	strb	r8, [r10, #TSK_USED_MATH]	@ set current->used_math
	add	r10, r10, #TSS_FPESAVE		@ r10 = workspace
	ldr	r4, .LC2
	ldr	pc, [r4]			@ Call FP emulator entry point

The kernel expects the emulator to return via one of two possible
points of return it passes to the emulator.  The emulator, if
successful in its emulation, jumps to ret_from_exception (passed in
r9) and the kernel takes care of returning control from the trap to
the user code.  If the emulator is unable to emulate the instruction,
it returns via _fpundefinstr (passed via lr) and the kernel halts the
user program with a core dump.

On entry to the emulator r10 points to an area of private FP workspace
reserved in the thread structure for this process.  This is where the
emulator saves its registers across calls.  The first word of this area
is used as a flag to detect the first time a process uses floating point,
so that the emulator startup cost can be avoided for tasks that don't
want it.

This routine does three things:

1) The kernel has created a struct pt_regs on the stack and saved the
user registers into it.  See /usr/include/asm/proc/ptrace.h for details.

2) It calls EmulateAll to emulate a floating point instruction.
EmulateAll returns 1 if the emulation was successful, or 0 if not.

3) If an instruction has been emulated successfully, it looks ahead at
the next instruction.  If it is a floating point instruction, it
executes the instruction, without returning to user space.  In this
way it repeatedly looks ahead and executes floating point instructions
until it encounters a non floating point instruction, at which time it
returns via _fpreturn.

This is done to reduce the effect of the trap overhead on each
floating point instructions.  GCC attempts to group floating point
instructions to allow the emulator to spread the cost of the trap over
several floating point instructions.  */

#include <asm/asm-offsets.h>

	.globl	nwfpe_enter
nwfpe_enter:
	mov	r4, lr			@ save the failure-return addresses
	mov	sl, sp			@ we access the registers via 'sl'

	ldr	r5, [sp, #S_PC]		@ get contents of PC;
	mov	r6, r0			@ save the opcode
emulate:
	ldr	r1, [sp, #S_PSR]	@ fetch the PSR
	bl	arm_check_condition	@ check the condition
	cmp	r0, #ARM_OPCODE_CONDTEST_PASS	@ condition passed?

	@ if condition code failed to match, next insn
	bne	next			@ get the next instruction;

	mov	r0, r6			@ prepare for EmulateAll()
	bl	EmulateAll		@ emulate the instruction
	cmp	r0, #0			@ was emulation successful
	reteq	r4			@ no, return failure

next:
	uaccess_enable r3
.Lx1:	ldrt	r6, [r5], #4		@ get the next instruction and
					@ increment PC
	uaccess_disable r3
	and	r2, r6, #0x0F000000	@ test for FP insns
	teq	r2, #0x0C000000
	teqne	r2, #0x0D000000
	teqne	r2, #0x0E000000
	retne	r9			@ return ok if not a fp insn

	str	r5, [sp, #S_PC]		@ update PC copy in regs

	mov	r0, r6			@ save a copy
	b	emulate			@ check condition and emulate

	@ We need to be prepared for the instructions at .Lx1 and .Lx2 
	@ to fault.  Emit the appropriate exception gunk to fix things up.
	@ ??? For some reason, faults can happen at .Lx2 even with a
	@ plain LDR instruction.  Weird, but it seems harmless.
	.pushsection .text.fixup,"ax"
	.align	2
.Lrep:	str     r4, [sp, #S_PC]		@ retry current instruction
.Lfix:	ret	r9			@ let the user eat segfaults
	.popsection

	.pushsection __ex_table,"a"
	.align	3
	.long	.Lx1, .Lfix
	.popsection

	@
	@ Check whether the instruction is a co-processor instruction.
	@ If yes, we need to call the relevant co-processor handler.
	@ Only FPE instructions are dispatched here, everything else
	@ is handled by undef hooks.
	@
	@ Emulators may wish to make use of the following registers:
	@  r4  = PC value to resume execution after successful emulation
	@  r9  = normal "successful" return address
	@  lr  = unrecognised instruction return address
	@ IRQs enabled, FIQs enabled.
	@
ENTRY(call_fpe)
	mov	r2, r4
	sub	r4, r4, #4			@ ARM instruction at user PC - 4
USERL(	.Lrep,	ldrt r0, [r4])			@ load opcode from user space
ARM_BE8(rev	r0, r0)				@ little endian instruction

	uaccess_disable ip

	get_thread_info r10			@ get current thread
	tst	r0, #0x08000000			@ only CDP/CPRT/LDC/STC have bit 27
	reteq	lr
	and	r8, r0, #0x00000f00		@ mask out CP number
#ifdef CONFIG_IWMMXT
	@ Test if we need to give access to iWMMXt coprocessors
	ldr	r5, [r10, #TI_FLAGS]
	rsbs	r7, r8, #(1 << 8)		@ CP 0 or 1 only
	movscs	r7, r5, lsr #(TIF_USING_IWMMXT + 1)
	movcs	r0, sp				@ pass struct pt_regs
	bcs	iwmmxt_task_enable
#endif
	add	pc, pc, r8, lsr #6
	nop

	ret	lr				@ CP#0
	b	do_fpe				@ CP#1 (FPE)
	b	do_fpe				@ CP#2 (FPE)
	ret	lr				@ CP#3
	ret	lr				@ CP#4
	ret	lr				@ CP#5
	ret	lr				@ CP#6
	ret	lr				@ CP#7
	ret	lr				@ CP#8
	ret	lr				@ CP#9
	ret	lr				@ CP#10 (VFP)
	ret	lr				@ CP#11 (VFP)
	ret	lr				@ CP#12
	ret	lr				@ CP#13
	ret	lr				@ CP#14 (Debug)
	ret	lr				@ CP#15 (Control)

do_fpe:
	add	r10, r10, #TI_FPSTATE		@ r10 = workspace
	ldr_va	pc, fp_enter, tmp=r4		@ Call FP module USR entry point

	@
	@ The FP module is called with these registers set:
	@  r0  = instruction
	@  r2  = PC+4
	@  r9  = normal "successful" return address
	@  r10 = FP workspace
	@  lr  = unrecognised FP instruction return address
	@

	.pushsection .data
	.align	2
ENTRY(fp_enter)
	.word	no_fp
	.popsection

no_fp:
	ret	lr
ENDPROC(no_fp)