/* SPDX-License-Identifier: GPL-2.0 */
/*
 * Non-emulated single-stepping support (currently limited to basic integer
 * computations) used to validate the instruction emulation infrastructure.
 *
 * Copyright (C) 2019 IBM Corporation
 */

#include <asm/asm-offsets.h>
#include <asm/ppc_asm.h>
#include <asm/code-patching-asm.h>
#include <linux/errno.h>

/* int exec_instr(struct pt_regs *regs) */
_GLOBAL(exec_instr)

        /*
         * Stack frame layout (INT_FRAME_SIZE bytes)
         *   In-memory pt_regs  (SP + STACK_INT_FRAME_REGS)
         *   Scratch space      (SP + 8)
         *   Back chain         (SP + 0)
         */

        /*
         * Allocate a new stack frame with enough space to hold the register
         * states in an in-memory pt_regs and also create the back chain to
         * the caller's stack frame.
         */
        stdu    r1, -INT_FRAME_SIZE(r1)

        /*
         * Save non-volatile GPRs on stack. This includes TOC pointer (GPR2)
         * and local variables (GPR14 to GPR31). The register for the pt_regs
         * parameter (GPR3) is saved additionally to ensure that the resulting
         * register state can still be saved even if GPR3 gets overwritten
         * when loading the initial register state for the test instruction.
         * The stack pointer (GPR1) and the thread pointer (GPR13) are not
         * saved as these should not be modified anyway.
         */
        SAVE_GPRS(2, 3, r1)
        SAVE_NVGPRS(r1)

        /*
         * Save LR on stack to ensure that the return address is available
         * even if it gets overwritten by the test instruction.
         */
        mflr    r0
        std     r0, _LINK(r1)

        /*
         * Save CR on stack. For simplicity, the entire register is saved
         * even though only fields 2 to 4 are non-volatile.
         */
        mfcr    r0
        std     r0, _CCR(r1)

        /*
         * Load register state for the test instruction without touching the
         * critical non-volatile registers. The register state is passed as a
         * pointer to a pt_regs instance.
         */
        subi    r31, r3, GPR0

        /* Load LR from pt_regs */
        ld      r0, _LINK(r31)
        mtlr    r0

        /* Load CR from pt_regs */
        ld      r0, _CCR(r31)
        mtcr    r0

        /* Load XER from pt_regs */
        ld      r0, _XER(r31)
        mtxer   r0

        /* Load GPRs from pt_regs */
        REST_GPR(0, r31)
        REST_GPRS(2, 12, r31)
        REST_NVGPRS(r31)

        /* Placeholder for the test instruction */
        .balign 64
1:      nop
        nop
        patch_site 1b patch__exec_instr

        /*
         * Since GPR3 is overwritten, temporarily restore it back to its
         * original state, i.e. the pointer to pt_regs, to ensure that the
         * resulting register state can be saved. Before doing this, a copy
         * of it is created in the scratch space which is used later on to
         * save it to pt_regs.
         */
        std     r3, 8(r1)
        REST_GPR(3, r1)

        /* Save resulting GPR state to pt_regs */
        subi    r3, r3, GPR0
        SAVE_GPR(0, r3)
        SAVE_GPR(2, r3)
        SAVE_GPRS(4, 12, r3)
        SAVE_NVGPRS(r3)

        /* Save resulting LR to pt_regs */
        mflr    r0
        std     r0, _LINK(r3)

        /* Save resulting CR to pt_regs */
        mfcr    r0
        std     r0, _CCR(r3)

        /* Save resulting XER to pt_regs */
        mfxer   r0
        std     r0, _XER(r3)

        /* Restore resulting GPR3 from scratch space and save it to pt_regs */
        ld      r0, 8(r1)
        std     r0, GPR3(r3)

        /* Set return value to denote execution success */
        li      r3, 0

        /* Continue */
        b       3f

        /* Set return value to denote execution failure */
2:      li      r3, -EFAULT

        /* Restore the non-volatile GPRs from stack */
3:      REST_GPR(2, r1)
        REST_NVGPRS(r1)

        /* Restore LR from stack to be able to return */
        ld      r0, _LINK(r1)
        mtlr    r0

        /* Restore CR from stack */
        ld      r0, _CCR(r1)
        mtcr    r0

        /* Tear down stack frame */
        addi    r1, r1, INT_FRAME_SIZE

        /* Return */
        blr

        /* Setup exception table */
        EX_TABLE(1b, 2b)

_ASM_NOKPROBE_SYMBOL(exec_instr)