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Commit 587064b6 authored by Punit Agrawal's avatar Punit Agrawal Committed by Will Deacon
Browse files

arm64: Add framework for legacy instruction emulation 



Typically, providing support for legacy instructions requires
emulating the behaviour of instructions whose encodings have become
undefined. If the instructions haven't been removed from the
architecture, there maybe an option in the implementation to turn
on/off the support for these instructions.

Create common infrastructure to support legacy instruction
emulation. In addition to emulation, also provide an option to support
hardware execution when supported. The default execution mode (one of
undef, emulate, hw exeuction) is dependent on the state of the
instruction (deprecated or obsolete) in the architecture and
can specified at the time of registering the instruction handlers. The
runtime state of the emulation can be controlled by writing to
individual nodes in sysctl. The expected default behaviour is
documented as part of this patch.

Reviewed-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: default avatarPunit Agrawal <punit.agrawal@arm.com>
Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
parent 0be0e44c

Documentation/arm64/legacy_instructions.txt

0 → 100644
+33 −0
Original line number Diff line number Diff line 
The arm64 port of the Linux kernel provides infrastructure to support

emulation of instructions which have been deprecated, or obsoleted in

the architecture. The infrastructure code uses undefined instruction

hooks to support emulation. Where available it also allows turning on

the instruction execution in hardware.



The emulation mode can be controlled by writing to sysctl nodes

(/proc/sys/abi). The following explains the different execution

behaviours and the corresponding values of the sysctl nodes -



* Undef

  Value: 0

  Generates undefined instruction abort. Default for instructions that

  have been obsoleted in the architecture, e.g., SWP



* Emulate

  Value: 1

  Uses software emulation. To aid migration of software, in this mode

  usage of emulated instruction is traced as well as rate limited

  warnings are issued. This is the default for deprecated

  instructions, .e.g., CP15 barriers



* Hardware Execution

  Value: 2

  Although marked as deprecated, some implementations may support the

  enabling/disabling of hardware support for the execution of these

  instructions. Using hardware execution generally provides better

  performance, but at the loss of ability to gather runtime statistics

  about the use of the deprecated instructions.



The default mode depends on the status of the instruction in the

architecture. Deprecated instructions should default to emulation

while obsolete instructions must be undefined by default.

arch/arm64/Kconfig

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@@ -164,6 +164,24 @@ config ARCH_XGENE 
	help

	  This enables support for AppliedMicro X-Gene SOC Family



comment "Processor Features"



menuconfig ARMV8_DEPRECATED

	bool "Emulate deprecated/obsolete ARMv8 instructions"

	depends on COMPAT

	help

	  Legacy software support may require certain instructions

	  that have been deprecated or obsoleted in the architecture.



	  Enable this config to enable selective emulation of these

	  features.



	  If unsure, say Y



if ARMV8_DEPRECATED



endif



endmenu



menu "Bus support"

arch/arm64/kernel/Makefile

+1 −0
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@@ -32,6 +32,7 @@ arm64-obj-$(CONFIG_JUMP_LABEL) += jump_label.o 
arm64-obj-$(CONFIG_KGDB)		+= kgdb.o

arm64-obj-$(CONFIG_EFI)			+= efi.o efi-stub.o efi-entry.o

arm64-obj-$(CONFIG_PCI)			+= pci.o

arm64-obj-$(CONFIG_ARMV8_DEPRECATED)	+= armv8_deprecated.o



obj-y					+= $(arm64-obj-y) vdso/

obj-m					+= $(arm64-obj-m)

arch/arm64/kernel/armv8_deprecated.c

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/*

 *  Copyright (C) 2014 ARM Limited

 *

 * This program is free software; you can redistribute it and/or modify

 * it under the terms of the GNU General Public License version 2 as

 * published by the Free Software Foundation.

 */



#include <linux/init.h>

#include <linux/list.h>

#include <linux/slab.h>

#include <linux/sysctl.h>



#include <asm/traps.h>



/*

 * The runtime support for deprecated instruction support can be in one of

 * following three states -

 *

 * 0 = undef

 * 1 = emulate (software emulation)

 * 2 = hw (supported in hardware)

 */

enum insn_emulation_mode {

	INSN_UNDEF,

	INSN_EMULATE,

	INSN_HW,

};



enum legacy_insn_status {

	INSN_DEPRECATED,

	INSN_OBSOLETE,

};



struct insn_emulation_ops {

	const char		*name;

	enum legacy_insn_status	status;

	struct undef_hook	*hooks;

	int			(*set_hw_mode)(bool enable);

};



struct insn_emulation {

	struct list_head node;

	struct insn_emulation_ops *ops;

	int current_mode;

	int min;

	int max;

};



static LIST_HEAD(insn_emulation);

static int nr_insn_emulated;

static DEFINE_RAW_SPINLOCK(insn_emulation_lock);



static void register_emulation_hooks(struct insn_emulation_ops *ops)

{

	struct undef_hook *hook;



	BUG_ON(!ops->hooks);



	for (hook = ops->hooks; hook->instr_mask; hook++)

		register_undef_hook(hook);



	pr_notice("Registered %s emulation handler\n", ops->name);

}



static void remove_emulation_hooks(struct insn_emulation_ops *ops)

{

	struct undef_hook *hook;



	BUG_ON(!ops->hooks);



	for (hook = ops->hooks; hook->instr_mask; hook++)

		unregister_undef_hook(hook);



	pr_notice("Removed %s emulation handler\n", ops->name);

}



static int update_insn_emulation_mode(struct insn_emulation *insn,

				       enum insn_emulation_mode prev)

{

	int ret = 0;



	switch (prev) {

	case INSN_UNDEF: /* Nothing to be done */

		break;

	case INSN_EMULATE:

		remove_emulation_hooks(insn->ops);

		break;

	case INSN_HW:

		if (insn->ops->set_hw_mode) {

			insn->ops->set_hw_mode(false);

			pr_notice("Disabled %s support\n", insn->ops->name);

		}

		break;

	}



	switch (insn->current_mode) {

	case INSN_UNDEF:

		break;

	case INSN_EMULATE:

		register_emulation_hooks(insn->ops);

		break;

	case INSN_HW:

		if (insn->ops->set_hw_mode && insn->ops->set_hw_mode(true))

			pr_notice("Enabled %s support\n", insn->ops->name);

		else

			ret = -EINVAL;

		break;

	}



	return ret;

}



static void register_insn_emulation(struct insn_emulation_ops *ops)

{

	unsigned long flags;

	struct insn_emulation *insn;



	insn = kzalloc(sizeof(*insn), GFP_KERNEL);

	insn->ops = ops;

	insn->min = INSN_UNDEF;



	switch (ops->status) {

	case INSN_DEPRECATED:

		insn->current_mode = INSN_EMULATE;

		insn->max = INSN_HW;

		break;

	case INSN_OBSOLETE:

		insn->current_mode = INSN_UNDEF;

		insn->max = INSN_EMULATE;

		break;

	}



	raw_spin_lock_irqsave(&insn_emulation_lock, flags);

	list_add(&insn->node, &insn_emulation);

	nr_insn_emulated++;

	raw_spin_unlock_irqrestore(&insn_emulation_lock, flags);



	/* Register any handlers if required */

	update_insn_emulation_mode(insn, INSN_UNDEF);

}



static int emulation_proc_handler(struct ctl_table *table, int write,

				  void __user *buffer, size_t *lenp,

				  loff_t *ppos)

{

	int ret = 0;

	struct insn_emulation *insn = (struct insn_emulation *) table->data;

	enum insn_emulation_mode prev_mode = insn->current_mode;



	table->data = &insn->current_mode;

	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);



	if (ret || !write || prev_mode == insn->current_mode)

		goto ret;



	ret = update_insn_emulation_mode(insn, prev_mode);

	if (!ret) {

		/* Mode change failed, revert to previous mode. */

		insn->current_mode = prev_mode;

		update_insn_emulation_mode(insn, INSN_UNDEF);

	}

ret:

	table->data = insn;

	return ret;

}



static struct ctl_table ctl_abi[] = {

	{

		.procname = "abi",

		.mode = 0555,

	},

	{ }

};



static void register_insn_emulation_sysctl(struct ctl_table *table)

{

	unsigned long flags;

	int i = 0;

	struct insn_emulation *insn;

	struct ctl_table *insns_sysctl, *sysctl;



	insns_sysctl = kzalloc(sizeof(*sysctl) * (nr_insn_emulated + 1),

			      GFP_KERNEL);



	raw_spin_lock_irqsave(&insn_emulation_lock, flags);

	list_for_each_entry(insn, &insn_emulation, node) {

		sysctl = &insns_sysctl[i];



		sysctl->mode = 0644;

		sysctl->maxlen = sizeof(int);



		sysctl->procname = insn->ops->name;

		sysctl->data = insn;

		sysctl->extra1 = &insn->min;

		sysctl->extra2 = &insn->max;

		sysctl->proc_handler = emulation_proc_handler;

		i++;

	}

	raw_spin_unlock_irqrestore(&insn_emulation_lock, flags);



	table->child = insns_sysctl;

	register_sysctl_table(table);

}



/*

 * Invoked as late_initcall, since not needed before init spawned.

 */

static int __init armv8_deprecated_init(void)

{

	register_insn_emulation_sysctl(ctl_abi);



	return 0;

}



late_initcall(armv8_deprecated_init);