Date:   Thu, 5 Jan 2023 13:55:26 +0100
From:   Clément Léger <clement.leger@...tlin.com>
To:     Yann Sionneau <ysionneau@...ray.eu>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org,
        Guillaume Thouvenin <gthouvenin@...ray.eu>,
        Julian Vetter <jvetter@...ray.eu>,
        Marius Gligor <mgligor@...ray.eu>
Subject: Re: [RFC PATCH 21/25] kvx: Add support for ftrace

Le Tue,  3 Jan 2023 17:43:55 +0100,
Yann Sionneau <ysionneau@...ray.eu> a écrit :

> Add support for ftrace to kvx arch.
> 
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Masami Hiramatsu <mhiramat@...nel.org>
> CC: Mark Rutland <mark.rutland@....com>
> CC: linux-kernel@...r.kernel.org
> Co-developed-by: Clement Leger <clement.leger@...tlin.com>
> Signed-off-by: Clement Leger <clement.leger@...tlin.com>
> Co-developed-by: Guillaume Thouvenin <gthouvenin@...ray.eu>
> Signed-off-by: Guillaume Thouvenin <gthouvenin@...ray.eu>
> Co-developed-by: Julian Vetter <jvetter@...ray.eu>
> Signed-off-by: Julian Vetter <jvetter@...ray.eu>
> Co-developed-by: Marius Gligor <mgligor@...ray.eu>
> Signed-off-by: Marius Gligor <mgligor@...ray.eu>
> Co-developed-by: Yann Sionneau <ysionneau@...ray.eu>
> Signed-off-by: Yann Sionneau <ysionneau@...ray.eu>
> ---
>  arch/kvx/include/asm/ftrace.h    |  41 ++++
>  arch/kvx/kernel/ftrace.c         | 339 ++++++++++++++++++++++++++++++
>  arch/kvx/kernel/mcount.S         | 340 +++++++++++++++++++++++++++++++
>  arch/kvx/kernel/return_address.c |  55 +++++
>  4 files changed, 775 insertions(+)
>  create mode 100644 arch/kvx/include/asm/ftrace.h
>  create mode 100644 arch/kvx/kernel/ftrace.c
>  create mode 100644 arch/kvx/kernel/mcount.S
>  create mode 100644 arch/kvx/kernel/return_address.c
> 
> diff --git a/arch/kvx/include/asm/ftrace.h b/arch/kvx/include/asm/ftrace.h
> new file mode 100644
> index 000000000000..f7bcb325d255
> --- /dev/null
> +++ b/arch/kvx/include/asm/ftrace.h
> @@ -0,0 +1,41 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2017-2023 Kalray Inc.
> + * Author(s): Guillaume Thouvenin
> + *            Clement Leger
> + */
> +
> +#ifndef _ASM_KVX_FTRACE_H
> +#define _ASM_KVX_FTRACE_H
> +
> +#include <asm/insns_defs.h>
> +
> +#define INSN_MAKE_IMM64_SYLLABLE_SIZE  INSN_SIZE(MAKE_IMM64)
> +#define INSN_ICALL_SYLLABLE_SIZE       INSN_SIZE(ICALL)
> +#define INSN_IGOTO_SYLLABLE_SIZE       INSN_SIZE(IGOTO)
> +#define INSN_CALL_SYLLABLE_SIZE        INSN_SIZE(CALL)
> +#define INSN_NOP_SYLLABLE_SIZE         INSN_SIZE(NOP)
> +
> +#define INSN_ICALL_REG_MASK  0x3f
> +
> +#define MCOUNT_ADDR	((unsigned long)(__mcount))
> +#define MCOUNT_INSN_SIZE   INSN_CALL_SYLLABLE_SIZE /* sizeof mcount call */
> +
> +extern void ftrace_graph_call(void);
> +
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +extern unsigned long ftrace_call_adjust(unsigned long addr);
> +struct dyn_arch_ftrace {
> +	unsigned int insn;
> +};
> +#endif
> +
> +extern void *return_address(unsigned int level);
> +#define ftrace_return_address(n) return_address(n)
> +
> +#ifdef CONFIG_FUNCTION_TRACER
> +extern void __mcount(void);
> +#define mcount __mcount
> +#endif /* CONFIG_FUNCTION_TRACER */
> +
> +#endif /* _ASM_KVX_FTRACE_H */
> diff --git a/arch/kvx/kernel/ftrace.c b/arch/kvx/kernel/ftrace.c
> new file mode 100644
> index 000000000000..4c9e3ef62714
> --- /dev/null
> +++ b/arch/kvx/kernel/ftrace.c
> @@ -0,0 +1,339 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2017-2023 Kalray Inc.
> + * Author(s): Guillaume Thouvenin
> + *            Marius Gligor
> + *            Clement Leger
> + */
> +
> +#include <linux/ftrace.h>
> +#include <linux/atomic.h>
> +#include <linux/stop_machine.h>
> +#include <asm/insns.h>
> +#include <asm/insns_defs.h>
> +#include <asm/cacheflush.h>
> +
> +/* The longest insns we check are for the far call: make + icall */
> +#define MAX_SYLLABLES_TO_CHECK (KVX_INSN_MAKE_IMM64_SIZE + INSN_ICALL_SYLLABLE_SIZE)
> +
> +static int read_insns_and_check(u32 *insns, u8 insns_len, u32 *addr)
> +{
> +	u32 insns_read[MAX_SYLLABLES_TO_CHECK];
> +	int syllables = insns_len / KVX_INSN_SYLLABLE_WIDTH;
> +	int i;
> +
> +	if (syllables > MAX_SYLLABLES_TO_CHECK) {
> +		pr_err("%s: shouldn't have more than %d syllables to check\n",
> +		       __func__, MAX_SYLLABLES_TO_CHECK);
> +		return -EINVAL;
> +	}
> +
> +	if (kvx_insns_read(insns_read, insns_len, addr)) {
> +		pr_err("%s: error when trying to read syllable\n", __func__);
> +		return -EFAULT;
> +	}
> +
> +	for (i = 0; i < syllables; i++) {
> +		if (insns[i] != insns_read[i]) {
> +			pr_err("%s: Instruction verification failed at PC 0x%lx\n",
> +			       __func__,
> +			       (unsigned long)addr + i * KVX_INSN_SYLLABLE_WIDTH);
> +			pr_err("%s: \tExpect  0x%x\n", __func__, insns[i]);
> +			pr_err("%s: \tRead    0x%x\n", __func__, insns_read[i]);
> +			return -EFAULT;
> +		}
> +	}
> +
> +	return 0;
> +}

Hi Yann,

Is this still needed ? I'm guessing the instructions should always be
correctly written no ? If not, something probably went horribly wrong ;)

> +
> +static int write_insns_and_check(u32 *insns, u8 insns_len, u32 *insn_addr)
> +{
> +	int ret;
> +
> +	ret = kvx_insns_write_nostop(insns, insns_len, insn_addr);
> +	if (ret)
> +		return ret;
> +
> +	/* Check that what have been written is correct. */
> +	return read_insns_and_check(insns, insns_len, insn_addr);
> +}
> +
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr,
> +			   unsigned long frame_pointer)
> +{
> +	unsigned long return_hooker = (unsigned long)&return_to_handler;
> +	unsigned long old;
> +
> +	if (unlikely(atomic_read(&current->tracing_graph_pause)))
> +		return;
> +
> +	old = *parent;
> +	*parent = return_hooker;
> +
> +	if (function_graph_enter(old, self_addr, frame_pointer, NULL))
> +		*parent = old;
> +}
> +
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +int ftrace_enable_ftrace_graph_caller(void)
> +{
> +	unsigned int insn[KVX_INSN_MAKE_IMM64_SIZE + KVX_INSN_IGOTO_SIZE];
> +	void *ip = (void *)ftrace_call;
> +
> +	KVX_INSN_MAKE_IMM64(insn, KVX_INSN_PARALLEL_EOB, KVX_REG_R32,
> +			    (unsigned long)&ftrace_graph_call);
> +	KVX_INSN_IGOTO(&insn[KVX_INSN_MAKE_IMM64_SIZE],
> +		       KVX_INSN_PARALLEL_EOB,
> +		       KVX_REG_R32);
> +
> +	return write_insns_and_check(insn,
> +				     INSN_MAKE_IMM64_SYLLABLE_SIZE
> +				     + INSN_IGOTO_SYLLABLE_SIZE,
> +				     ip);
> +}
> +
> +int ftrace_disable_ftrace_graph_caller(void)
> +{
> +	unsigned int nop;
> +	void *ip = (void *)ftrace_call;
> +
> +	KVX_INSN_NOP(&nop, KVX_INSN_PARALLEL_EOB);
> +	return write_insns_and_check(&nop,
> +				     INSN_NOP_SYLLABLE_SIZE,
> +				     ip + INSN_MAKE_IMM64_SYLLABLE_SIZE);
> +}
> +#endif /* CONFIG_DYNAMIC_FTRACE */
> +
> +#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> +
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +struct kvx_ftrace_modify_param {
> +	atomic_t	cpu_ack;
> +	int		cpu_master;
> +	int		cmd;
> +};
> +
> +static int __ftrace_modify_code_kvx(void *data)
> +{
> +	struct kvx_ftrace_modify_param *mp = data;
> +	int no_cpus = num_online_cpus();
> +	int cpu = smp_processor_id();
> +
> +	if (cpu == mp->cpu_master) {
> +		ftrace_modify_all_code(mp->cmd);
> +
> +		/* Inform the other cpus that they can invalidate I-cache */
> +		atomic_inc(&mp->cpu_ack);
> +
> +		/* param (which contains the cpu_ack counter) is allocated on the
> +		 * master stack: the master must wait for all other CPUs to leave
> +		 * this function before returning and releasing the stack
> +		 * allocation.
> +		 */
> +		while (atomic_read(&mp->cpu_ack) < no_cpus)
> +			cpu_relax();
> +	} else {
> +		/* Wait for the master cpu to finish the code modification */
> +		while (atomic_read(&mp->cpu_ack) == 0)
> +			cpu_relax();
> +		atomic_inc(&mp->cpu_ack);
> +
> +		l1_inval_icache_all();
> +	}
> +
> +	return 0;
> +}
> +
> +void arch_ftrace_update_code(int command)
> +{
> +	const struct cpumask *cpumask = cpu_online_mask;
> +	struct kvx_ftrace_modify_param mp = {
> +		.cpu_ack = ATOMIC_INIT(0),
> +		.cpu_master = smp_processor_id(),
> +		.cmd = command,
> +	};
> +
> +	stop_machine(__ftrace_modify_code_kvx, &mp, cpu_online_mask);
> +}
> +
> +unsigned long ftrace_call_adjust(unsigned long addr)
> +{
> +	/*
> +	 * Module are using far call and kernel functions are using
> +	 * pcrel. If it is a call we don't need to adjust the address but
> +	 * if it is an icall the address is on the make. The generated code
> +	 * looks like:
> +	 *
> +	 * 1c:   e0 00 c4 8f                             get $r32 = $ra
> +	 * 20:   00 00 84 e0 00 00 00 80 00 00 00 00     make $r33 = 0 (0x0);;
> +	 *
> +	 *            20: R_KVX_S64_LO10      __mcount
> +	 *            24: R_KVX_S64_UP27      __mcount
> +	 *            28: R_KVX_S64_EX27      __mcount
> +	 * 2c:   21 00 dc 0f                             icall $r33;;
> +	 *
> +	 * So we just need to add INSN_MAKE_IMM64_SYLLABLE_SIZE (0xc) to the
> +	 * address.
> +	 */
> +	unsigned int insn;
> +
> +	/*
> +	 * The CALL is 1 syllable while the MAKE IMM64 is 3. But we just
> +	 * need to check that the first syllable of the MAKE IMM64 is the
> +	 * LO10. So finally we just need to read one syllable to adjust the
> +	 * call.
> +	 */
> +	if (kvx_insns_read(&insn, KVX_INSN_SYLLABLE_WIDTH, (void *)addr)) {
> +		pr_err("%s: error when trying to read syllable\n", __func__);
> +		return 0;
> +	}
> +
> +	if (IS_INSN(insn, CALL))
> +		return addr;
> +
> +	if (IS_INSN(insn, MAKE_IMM64))
> +		return addr + INSN_MAKE_IMM64_SYLLABLE_SIZE;
> +
> +	pr_err("%s: syllable is neither a CALL nor a MAKE\n", __func__);
> +	return 0;
> +}
> +
> +/*
> + * Do runtime patching of the active tracer.
> + * This will be modifying the assembly code at the location of the
> + * ftrace_call symbol inside of the ftrace_caller() function.
> + */
> +int ftrace_update_ftrace_func(ftrace_func_t func)
> +{
> +	void *ip;
> +	unsigned int insn[KVX_INSN_MAKE_IMM64_SIZE + KVX_INSN_ICALL_SIZE];
> +
> +	ip = (void *)ftrace_call;
> +	KVX_INSN_MAKE_IMM64(insn, KVX_INSN_PARALLEL_EOB, KVX_REG_R32,
> +			    (unsigned long)func);
> +	KVX_INSN_ICALL(&insn[KVX_INSN_MAKE_IMM64_SIZE],
> +		       KVX_INSN_PARALLEL_EOB,
> +		       KVX_REG_R32);
> +	return write_insns_and_check(insn,
> +				     INSN_MAKE_IMM64_SYLLABLE_SIZE
> +				     + INSN_ICALL_SYLLABLE_SIZE,
> +				     ip);
> +}
> +
> +/*
> + * Turn the mcount call site into a call to an arbitrary location (but
> + * typically that is ftrace_caller()) at runtime.
> + */
> +int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> +{
> +	void *ip = (void *)rec->ip;
> +	unsigned int insn;
> +	int ret;
> +
> +	/* Ensure that a NOP will be replaced */
> +	if (kvx_insns_read(&insn, KVX_INSN_SYLLABLE_WIDTH, ip)) {
> +		pr_err("%s: failed to read insn\n", __func__);
> +		return -EFAULT;
> +	}
> +
> +	if (!IS_INSN(insn, NOP)) {
> +		pr_err("%s: insn 0x%x is not a NOP\n", __func__, insn);
> +		return -EINVAL;
> +	}
> +
> +	/*
> +	 * Now we can replace the instruction depending of what has been
> +	 * nopified (call or icall)
> +	 */
> +	insn = rec->arch.insn;
> +
> +	if (IS_INSN(insn, CALL)) {
> +		s32 pcrel = addr - (unsigned long) ip;
> +		u32 insn_call;
> +
> +		BUG_ON(KVX_INSN_GOTO_PCREL27_CHECK(pcrel));
> +		KVX_INSN_CALL(&insn_call, KVX_INSN_PARALLEL_EOB, pcrel);
> +
> +		return write_insns_and_check(&insn_call,
> +					     INSN_CALL_SYLLABLE_SIZE, ip);
> +	}
> +
> +	if (IS_INSN(insn, ICALL)) {
> +		u32 insn_make[KVX_INSN_MAKE_IMM64_SIZE];
> +		u32 r = insn & INSN_ICALL_REG_MASK;
> +
> +		KVX_INSN_MAKE_IMM64(insn_make, KVX_INSN_PARALLEL_EOB, r, addr);
> +		ret = write_insns_and_check(insn_make,
> +					    INSN_MAKE_IMM64_SYLLABLE_SIZE,
> +					    ip - INSN_MAKE_IMM64_SYLLABLE_SIZE);
> +		if (ret)
> +			return ret;
> +
> +		return write_insns_and_check(&insn,
> +					      INSN_ICALL_SYLLABLE_SIZE, ip);
> +	}
> +
> +	/* It is neither a call nor an icall */
> +	pr_err("%s: insn 0x%x is neither a CALL nor ICALL\n", __func__, insn);
> +	return -EINVAL;
> +}
> +
> +/*
> + * Turn the mcount call site into a nop at runtime
> + */
> +int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
> +		    unsigned long addr)
> +{
> +	unsigned long ip = rec->ip;
> +	unsigned int insn;
> +	unsigned int nop;
> +
> +	/*
> +	 * Ensure that the instruction that will be replaced is a call or an
> +	 * icall to addr.
> +	 */
> +	if (kvx_insns_read(&insn, KVX_INSN_SYLLABLE_WIDTH, (void *)ip)) {
> +		pr_err("%s: error when trying to read syllable\n", __func__);
> +		return -EFAULT;
> +	}
> +
> +	if (IS_INSN(insn, CALL)) {
> +		int pcrel = ((int)(insn & 0x7ffffff) << 5) >> 3;

Hardcoded values

> +
> +		if ((ip + pcrel != addr)) {
> +			pr_err("%s: failed to check call addr 0x%lx != 0x%lx\n",
> +			       __func__, ip + pcrel, addr);
> +			return -EINVAL;
> +		}
> +	} else if (IS_INSN(insn, ICALL)) {
> +		unsigned int insn_make[KVX_INSN_MAKE_IMM64_SIZE];
> +		unsigned int reg = insn & INSN_ICALL_REG_MASK;
> +		int ret;
> +
> +		KVX_INSN_MAKE_IMM64(insn_make,
> +				    KVX_INSN_PARALLEL_EOB, reg,
> +				    addr);
> +
> +		ret = read_insns_and_check(insn_make,
> +					   INSN_MAKE_IMM64_SYLLABLE_SIZE,
> +					   (void *)ip -
> +					   INSN_MAKE_IMM64_SYLLABLE_SIZE);
> +		if (ret)
> +			return ret;
> +	} else {
> +		pr_err("%s: insn 0x%x is neither a CALL nor an ICALL\n",
> +		       __func__, insn);
> +		return -EINVAL;
> +	}
> +
> +	rec->arch.insn = insn;
> +	KVX_INSN_NOP(&nop, KVX_INSN_PARALLEL_EOB);
> +	return write_insns_and_check(&nop, INSN_NOP_SYLLABLE_SIZE, (void *)ip);
> +}
> +
> +#endif /* CONFIG_DYNAMIC_FTRACE */
> +
> +/* __mcount is defined in mcount.S */
> +EXPORT_SYMBOL(__mcount);
> diff --git a/arch/kvx/kernel/mcount.S b/arch/kvx/kernel/mcount.S
> new file mode 100644
> index 000000000000..317797aeed42
> --- /dev/null
> +++ b/arch/kvx/kernel/mcount.S
> @@ -0,0 +1,340 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2017-2023 Kalray Inc.
> + * Author(s): Guillaume Thouvenin
> + *            Marius Gligor
> + *            Clement Leger
> + */
> +
> +#include <linux/linkage.h>
> +#include <asm/insns_defs.h>
> +#include <asm/sfr_defs.h>
> +
> +.altmacro
> +
> +/*
> + * !!! The call to __mcount is special and breaks the ABI !!!
> + * It is because with dynamic ftrace the call can be replaced by any
> + * tracer and GCC cannot assume which caller registers must be saved.
> + * It's because the call is being inserted in the prologue of functions
> + * that is out of GCC knowledge.
> + * We could make this known to GCC, but then we would hit the problem of not
> + * being able to easily decide which insns are emitted and how they are
> + * bundled. So we choose to save nothing when calling __mcount and we will
> + * save all caller registers once __mcount called.
> + *
> + * Another specificity is that parameter is passed in $r32 instead of $r0.
> + *
> + * Our stack will be:
> + *	0  | $FP $RA $r15 -
> + *	32 | $r0 $r1 $r2 $r3
> + *	64 | $r4 $r5 $r6 $r7
> + *	96 | $r8 $r9 $r10 $r11
> + *	128| $r32 $r33 $r34 $r35
> + *	160| $r36 $r37 $r38 $r39
> + *	192| $r40 $r41 $r42 $r43
> + *	224| $r44 $r45 $r46 $r47
> + *	256| $r48 $r49 $r50 $r51
> + *	288| $r52 $r53 $r54 $r55
> + *	320| $r56 $r57 $r58 $r59
> + *	352| $r60 $r61 $r62 $r63
> + *	384| $r16 $r17
> + */
> +
> +#define STACK_OFFSET_FP   0
> +#define STACK_OFFSET_RA   (STACK_OFFSET_FP + 8)
> +#define STACK_OFFSET_R15  (STACK_OFFSET_RA + 8)
> +#define STACK_OFFSET_Q0   (STACK_OFFSET_FP + 32)
> +#define STACK_OFFSET_Q4   (STACK_OFFSET_Q0 + 32)
> +#define STACK_OFFSET_Q8   (STACK_OFFSET_Q4 + 32)
> +#define STACK_OFFSET_Q32  (STACK_OFFSET_Q8 + 32)
> +#define STACK_OFFSET_Q36  (STACK_OFFSET_Q32 + 32)
> +#define STACK_OFFSET_Q40  (STACK_OFFSET_Q36 + 32)
> +#define STACK_OFFSET_Q44  (STACK_OFFSET_Q40 + 32)
> +#define STACK_OFFSET_Q48  (STACK_OFFSET_Q44 + 32)
> +#define STACK_OFFSET_Q52  (STACK_OFFSET_Q48 + 32)
> +#define STACK_OFFSET_Q56  (STACK_OFFSET_Q52 + 32)
> +#define STACK_OFFSET_Q60  (STACK_OFFSET_Q56 + 32)
> +#define STACK_OFFSET_P16  (STACK_OFFSET_Q60 + 32)
> +#define STACK_SIZE        (STACK_OFFSET_P16 + 16)
> +
> +#define RA_DWARF_REGNO    (64 + KVX_SFR_RA)
> +#define R14_DWARF_REGNO   14
> +
> +.macro save_all_context_but_P16
> +	/*
> +	 * r16 and r17 are already saved. r16 is already used to store the
> +	 * address of the function tracer if any.
> +	 * so !!! USE r17 as scratch register !!!
> +	 */
> +	get $r17 = $ra
> +	sd STACK_OFFSET_FP[$r12] = $r14
> +	;;
> +	.cfi_offset R14_DWARF_REGNO, -STACK_SIZE + STACK_OFFSET_FP
> +	sd STACK_OFFSET_RA[$r12] = $r17
> +	;;
> +	.cfi_offset RA_DWARF_REGNO, -STACK_SIZE + STACK_OFFSET_RA
> +	sd STACK_OFFSET_R15[$r12] = $r15
> +	;;
> +	so STACK_OFFSET_Q0[$r12] = $r0r1r2r3
> +	;;
> +	so STACK_OFFSET_Q4[$r12] = $r4r5r6r7
> +	;;
> +	so STACK_OFFSET_Q8[$r12] = $r8r9r10r11
> +	;;
> +	so STACK_OFFSET_Q32[$r12] = $r32r33r34r35
> +	;;
> +	so STACK_OFFSET_Q36[$r12] = $r36r37r38r39
> +	;;
> +	so STACK_OFFSET_Q40[$r12] = $r40r41r42r43
> +	;;
> +	so STACK_OFFSET_Q44[$r12] = $r44r45r46r47
> +	;;
> +	so STACK_OFFSET_Q48[$r12] = $r48r49r50r51
> +	;;
> +	so STACK_OFFSET_Q52[$r12] = $r52r53r54r55
> +	;;
> +	so STACK_OFFSET_Q56[$r12] = $r56r57r58r59
> +	;;
> +	so STACK_OFFSET_Q60[$r12] = $r60r61r62r63
> +	;;
> +.endm
> +
> +.macro restore_all_context
> +	ld $r15 = STACK_OFFSET_RA[$r12]
> +	;;
> +	ld $r14 = STACK_OFFSET_FP[$r12]
> +	set $ra = $r15
> +	;;
> +	.cfi_restore RA_DWARF_REGNO
> +	.cfi_restore R14_DWARF_REGNO
> +	ld $r15 = STACK_OFFSET_R15[$r12]
> +	;;
> +	lq $r16r17 = STACK_OFFSET_P16[$r12]
> +	;;
> +	lo $r0r1r2r3 = STACK_OFFSET_Q0[$r12]
> +	;;
> +	lo $r4r5r6r7 = STACK_OFFSET_Q4[$r12]
> +	;;
> +	lo $r8r9r10r11 = STACK_OFFSET_Q8[$r12]
> +	;;
> +	lo $r32r33r34r35 = STACK_OFFSET_Q32[$r12]
> +	;;
> +	lo $r36r37r38r39 = STACK_OFFSET_Q36[$r12]
> +	;;
> +	lo $r40r41r42r43 = STACK_OFFSET_Q40[$r12]
> +	;;
> +	lo $r44r45r46r47 = STACK_OFFSET_Q44[$r12]
> +	;;
> +	lo $r48r49r50r51 = STACK_OFFSET_Q48[$r12]
> +	;;
> +	lo $r52r53r54r55 = STACK_OFFSET_Q52[$r12]
> +	;;
> +	lo $r56r57r58r59 = STACK_OFFSET_Q56[$r12]
> +	;;
> +	lo $r60r61r62r63 = STACK_OFFSET_Q60[$r12]
> +	;;
> +	addd $r12 = $r12, STACK_SIZE
> +	;;
> +	.cfi_def_cfa_offset 0
> +.endm
> +
> +ENTRY(__mcount)
> +.cfi_startproc
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +	/*
> +	 * With dynamic ftrace mcount() is only used during boot and then all
> +	 * references to it will be patched out never to return. ftrace_caller
> +	 * is used.
> +	 */
> +	ret
> +	;;
> +#endif
> +	addd $r12 = $r12, -STACK_SIZE
> +	;;
> +	.cfi_def_cfa_offset STACK_SIZE
> +	sq STACK_OFFSET_P16[$r12] = $r16r17
> +	;;
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +	/*
> +	 * Check the function pointers ftrace_graph_return
> +	 * (compare to ftrace_stub) and ftrace_graph_entry (compare to
> +	 * ftrace_graph_entry_stub).  If either of those is not set to the
> +	 * relevant stub function, call the arch-specific function
> +	 * ftrace_graph_caller.
> +	 */
> +	make $r16 = ftrace_graph_return
> +	make $r17 = ftrace_graph_entry
> +	;;
> +	ld $r16 = 0[$r16]
> +	;;
> +	compd.eq $r16 = $r16, ftrace_stub
> +	ld $r17 = 0[$r17]
> +	;;
> +	/*
> +	 * If ftrace_graph_return is not equal to ftrace_stub ($r16 == 0) then
> +	 * jump to ftrace_graph_caller. Otherwise compare ftrace_graph_entry.
> +	 */
> +	cb.deqz $r16? ftrace_graph_caller
> +	compd.eq $r17 = $r17, ftrace_graph_entry_stub
> +	;;
> +	/*
> +	 * If ftrace_graph_entry is equal to ftrace_graph_entry_stub ($r17 == 1)
> +	 * then jump to ftrace_graph_end. If not we continue.
> +	 */
> +	cb.dnez $r17? ftrace_graph_end
> +	;;
> +ftrace_graph_caller:
> +	save_all_context_but_P16
> +	;;
> +GLOBAL(ftrace_graph_call)
> +	ld $r16 = 0[$r12]
> +	;;
> +	addd $r0 = $r16, 0x8 /* a pointer to the frompc */
> +	get $r1 = $ra /* selfpc */
> +	copyd $r2 = $r16 /* FP of the function that called __mcount */
> +	;;
> +	/* get the address of the bundle */
> +	addd $r1 = $r1, -KVX_INSN_SYLLABLE_WIDTH
> +	;;
> +	call prepare_ftrace_return
> +	;;
> +	restore_all_context
> +	;;
> +	ret
> +	;;
> +ftrace_graph_end:
> +	.cfi_def_cfa_offset STACK_SIZE
> +#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> +	/*
> +	 * Check if function pointer ftrace_trace_function is set to
> +	 * ftrace_stub.
> +	 * If it isn't, then call that function in the same way the mcount
> +	 * function normally calls __mcount_internal
> +	 * - the first argument is the "frompc" ($r0)
> +	 * - the second argument is the "selfpc" ($ra - mcount size adjustement)
> +	 * As we don't use any callee saved registers we don't need to save
> +	 * them.
> +	 */
> +	make $r16 = ftrace_trace_function
> +	;;
> +	ld $r16 = 0[$r16]
> +	;;
> +	/*
> +	 * NOTE: keep value in $r16 (the function we may call if set). See
> +	 *       comment in save_all_context_but_P16.
> +	 */
> +	compd.eq $r17 = $r16, ftrace_stub
> +	;;
> +	/*
> +	 * If ftrace_trace_function is equal to ftrace_stub ($r17 == 1), there
> +	 * is nothing to do, so return immediately otherwise call the function
> +	 * in $r16 with correct parameters.
> +	 */
> +	cb.deqz $r17? 1f
> +	;;
> +	/* Only need to restore P16 */
> +	lq $r16r17 = STACK_OFFSET_P16[$r12]
> +	;;
> +	addd $r12 = $r12, STACK_SIZE
> +	;;
> +	.cfi_def_cfa_offset 0
> +	ret
> +	;;
> +
> +1:
> +	save_all_context_but_P16
> +	;;
> +	copyd $r0 = $r32
> +	get $r1 = $ra
> +	;;
> +	addd $r1 = $r1, -KVX_INSN_SYLLABLE_WIDTH
> +	;;
> +	icall $r16
> +	;;
> +	restore_all_context
> +	;;
> +	ret
> +	;;
> +.cfi_endproc
> +ENDPROC(__mcount)
> +
> +/* Required by ftrace_graph_ret_addr */
> +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> +ENTRY(return_to_handler)
> +	addd $r12 = $r12, -128
> +	;;
> +	so 0[$r12] = $r0r1r2r3
> +	;;
> +	so 32[$r12] = $r4r5r6r7
> +	;;
> +	so 64[$r12] = $r8r9r10r11
> +	;;
> +	call ftrace_return_to_handler
> +	;;
> +	set $ra = $r0
> +	lo $r0r1r2r3 = 0[$r12]
> +	;;
> +	lo $r4r5r6r7 = 32[$r12]
> +	;;
> +	lo $r8r9r10r11 = 64[$r12]
> +	addd $r12 = $r12, 128
> +	;;
> +	ret
> +	;;
> +ENDPROC(return_to_handler)
> +#endif
> +
> +#ifdef CONFIG_DYNAMIC_FTRACE
> +ENTRY(ftrace_caller)
> +.cfi_startproc
> +
> +	addd $r12 = $r12, -STACK_SIZE
> +	;;
> +	.cfi_def_cfa_offset STACK_SIZE
> +	sq STACK_OFFSET_P16[$r12] = $r16r17
> +	;;
> +	save_all_context_but_P16
> +	;;
> +	copyd $r0 = $r32
> +	get $r1 = $ra
> +	;;
> +	addd $r1 = $r1, -KVX_INSN_SYLLABLE_WIDTH
> +	;;
> +	goto ftrace_call
> +	;;
> +.align 64
> +GLOBAL(ftrace_call)
> +	/*
> +	 * To work with module we need to use far call. So prepare the
> +	 * space for them:
> +	 *  - make: 3 syllables
> +	 *  - icall/igoto: 1 syllable
> +	 *
> +	 * We are using igoto in ftrace_enable_ftrace_graph_caller(). This is
> +	 * because this path is a tweak to enable function graph tracer with
> +	 * dynamic ftrace. We need to jump to ftrace_graph_call without updating
> +	 * RA and FP. Once in ftrace_graph_call we will return to the correct
> +	 * address from the classical path of the function graph tracer.
> +	 */
> +	nop
> +	;;
> +	nop
> +	;;
> +	nop
> +	;;
> +	nop
> +	;;
> +	restore_all_context
> +	;;
> +	ret
> +	;;
> +.cfi_endproc
> +ENDPROC(ftrace_caller)
> +#endif // CONFIG_DYNAMIC_FTRACE
> +
> +ENTRY(ftrace_stub)
> +	ret
> +	;;
> +ENDPROC(ftrace_stub)
> +
> diff --git a/arch/kvx/kernel/return_address.c b/arch/kvx/kernel/return_address.c
> new file mode 100644
> index 000000000000..3ddf7b79662c
> --- /dev/null
> +++ b/arch/kvx/kernel/return_address.c
> @@ -0,0 +1,55 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * derived from arch/arm64/kernel/return_address.c
> + *
> + * Copyright (C) 2017-2023 Kalray Inc.
> + * Author(s): Clement Leger
> + */
> +
> +#include <linux/export.h>
> +#include <linux/ftrace.h>
> +#include <linux/kprobes.h>
> +
> +#include <asm/stacktrace.h>
> +
> +struct return_address_data {
> +	unsigned int level;
> +	void *addr;
> +};
> +
> +static bool save_return_addr(unsigned long pc, void *d)
> +{
> +	struct return_address_data *data = d;
> +
> +	/* We hit the desired level, return the address */
> +	if (data->level == 0) {
> +		data->addr = (void *) pc;
> +		return true;
> +	}
> +
> +	data->level--;
> +	return false;
> +}
> +NOKPROBE_SYMBOL(save_return_addr);
> +
> +void *return_address(unsigned int level)
> +{
> +	struct return_address_data data;
> +	struct stackframe frame;
> +
> +	/* Skip this function + caller */
> +	data.level = level + 2;
> +	data.addr = NULL;
> +
> +	start_stackframe(&frame,
> +			 (unsigned long) __builtin_frame_address(0),
> +			 (unsigned long) return_address);
> +	walk_stackframe(current, &frame, save_return_addr, &data);
> +
> +	if (!data.level)
> +		return data.addr;
> +	else
> +		return NULL;
> +}
> +EXPORT_SYMBOL_GPL(return_address);
> +NOKPROBE_SYMBOL(return_address);



-- 
Clément Léger,
Embedded Linux and Kernel engineer at Bootlin
https://bootlin.com

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