Re: [PATCH v3] ARC: ARCv2: jump label: implement jump label patching

[Vineet Gupta]

On 11/8/19 8:20 AM, Eugeniy Paltsev wrote:
> Implement jump label patching for ARC. Jump labels provide
> an interface to generate dynamic branches using
> self-modifying code.
>
> This allows us to implement conditional branches where
> changing branch direction is expensive but branch selection
> is basically 'free'
>
> This implementation uses 32-bit NOP and BRANCH instructions
> which forced to be aligned by 4 to guarantee that they don't
> cross L1 cache line boundary / L1 I$ cache fetch block
> boundary and can be update atomically.

As confirmed by h/w folks, for ARC cache flush, the L1 fetch block is not really
relevant.

> Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@xxxxxxxxxxxx>

LGTM (I've removed the fetch block mention from changelog)

Added to for-next

Thx,
-Vineet

> ---
> Changes v1->v2:
>  * Patched instruction should not cross L1 I$ fetch block boundary and
>    not only L1 I$ line. Fix comments and asserts in code.
>  * Other small comments fix and code cleanup.
> Changes v2->v3:
>  * Code cleanup.
>  * Mark testdata array as __initconst
>  * Drop check about fetch block in code.
>  * Invert ARC_DBG_JUMP_LABEL vs STATIC_KEYS_SELFTEST dependency.
>
>  arch/arc/Kconfig                  |   8 ++
>  arch/arc/include/asm/cache.h      |   2 +
>  arch/arc/include/asm/jump_label.h |  72 +++++++++++++
>  arch/arc/kernel/Makefile          |   1 +
>  arch/arc/kernel/jump_label.c      | 170 ++++++++++++++++++++++++++++++
>  5 files changed, 253 insertions(+)
>  create mode 100644 arch/arc/include/asm/jump_label.h
>  create mode 100644 arch/arc/kernel/jump_label.c
>
> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> index 8383155c8c82..375f9d278139 100644
> --- a/arch/arc/Kconfig
> +++ b/arch/arc/Kconfig
> @@ -46,6 +46,7 @@ config ARC
>  	select OF_EARLY_FLATTREE
>  	select PCI_SYSCALL if PCI
>  	select PERF_USE_VMALLOC if ARC_CACHE_VIPT_ALIASING
> +	select HAVE_ARCH_JUMP_LABEL if ISA_ARCV2 && !CPU_ENDIAN_BE32
>  
>  config ARCH_HAS_CACHE_LINE_SIZE
>  	def_bool y
> @@ -525,6 +526,13 @@ config ARC_DW2_UNWIND
>  config ARC_DBG_TLB_PARANOIA
>  	bool "Paranoia Checks in Low Level TLB Handlers"
>  
> +config ARC_DBG_JUMP_LABEL
> +	bool "Paranoid checks in Static Keys (jump labels) code"
> +	depends on JUMP_LABEL
> +	default y if STATIC_KEYS_SELFTEST
> +	help
> +	  Enable paranoid checks and self-test of both ARC-specific and generic
> +	  part of static keys (jump labels) related code.
>  endif
>  
>  config ARC_BUILTIN_DTB_NAME
> diff --git a/arch/arc/include/asm/cache.h b/arch/arc/include/asm/cache.h
> index 918804c7c1a4..d8ece4292388 100644
> --- a/arch/arc/include/asm/cache.h
> +++ b/arch/arc/include/asm/cache.h
> @@ -25,6 +25,8 @@
>  
>  #ifndef __ASSEMBLY__
>  
> +#include <linux/build_bug.h>
> +
>  /* Uncached access macros */
>  #define arc_read_uncached_32(ptr)	\
>  ({					\
> diff --git a/arch/arc/include/asm/jump_label.h b/arch/arc/include/asm/jump_label.h
> new file mode 100644
> index 000000000000..9d9618079739
> --- /dev/null
> +++ b/arch/arc/include/asm/jump_label.h
> @@ -0,0 +1,72 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_ARC_JUMP_LABEL_H
> +#define _ASM_ARC_JUMP_LABEL_H
> +
> +#ifndef __ASSEMBLY__
> +
> +#include <linux/stringify.h>
> +#include <linux/types.h>
> +
> +#define JUMP_LABEL_NOP_SIZE 4
> +
> +/*
> + * NOTE about '.balign 4':
> + *
> + * To make atomic update of patched instruction available we need to guarantee
> + * that this instruction doesn't cross L1 cache line boundary.
> + *
> + * As of today we simply align instruction which can be patched by 4 byte using
> + * ".balign 4" directive. In that case patched instruction is aligned with one
> + * 16-bit NOP_S if this is required.
> + * However 'align by 4' directive is much stricter than it actually required.
> + * It's enough that our 32-bit instruction don't cross L1 cache line boundary /
> + * L1 I$ fetch block boundary which can be achieved by using
> + * ".bundle_align_mode" assembler directive. That will save us from adding
> + * useless NOP_S padding in most of the cases.
> + *
> + * TODO: switch to ".bundle_align_mode" directive using whin it will be
> + * supported by ARC toolchain.
> + */
> +
> +static __always_inline bool arch_static_branch(struct static_key *key,
> +					       bool branch)
> +{
> +	asm_volatile_goto(".balign "__stringify(JUMP_LABEL_NOP_SIZE)"	\n"
> +		 "1:							\n"
> +		 "nop							\n"
> +		 ".pushsection __jump_table, \"aw\"			\n"
> +		 ".word 1b, %l[l_yes], %c0				\n"
> +		 ".popsection						\n"
> +		 : : "i" (&((char *)key)[branch]) : : l_yes);
> +
> +	return false;
> +l_yes:
> +	return true;
> +}
> +
> +static __always_inline bool arch_static_branch_jump(struct static_key *key,
> +						    bool branch)
> +{
> +	asm_volatile_goto(".balign "__stringify(JUMP_LABEL_NOP_SIZE)"	\n"
> +		 "1:							\n"
> +		 "b %l[l_yes]						\n"
> +		 ".pushsection __jump_table, \"aw\"			\n"
> +		 ".word 1b, %l[l_yes], %c0				\n"
> +		 ".popsection						\n"
> +		 : : "i" (&((char *)key)[branch]) : : l_yes);
> +
> +	return false;
> +l_yes:
> +	return true;
> +}
> +
> +typedef u32 jump_label_t;
> +
> +struct jump_entry {
> +	jump_label_t code;
> +	jump_label_t target;
> +	jump_label_t key;
> +};
> +
> +#endif  /* __ASSEMBLY__ */
> +#endif
> diff --git a/arch/arc/kernel/Makefile b/arch/arc/kernel/Makefile
> index de6251132310..e784f5396dda 100644
> --- a/arch/arc/kernel/Makefile
> +++ b/arch/arc/kernel/Makefile
> @@ -20,6 +20,7 @@ obj-$(CONFIG_ARC_EMUL_UNALIGNED) 	+= unaligned.o
>  obj-$(CONFIG_KGDB)			+= kgdb.o
>  obj-$(CONFIG_ARC_METAWARE_HLINK)	+= arc_hostlink.o
>  obj-$(CONFIG_PERF_EVENTS)		+= perf_event.o
> +obj-$(CONFIG_JUMP_LABEL)		+= jump_label.o
>  
>  obj-$(CONFIG_ARC_FPU_SAVE_RESTORE)	+= fpu.o
>  CFLAGS_fpu.o   += -mdpfp
> diff --git a/arch/arc/kernel/jump_label.c b/arch/arc/kernel/jump_label.c
> new file mode 100644
> index 000000000000..b8600dc325b5
> --- /dev/null
> +++ b/arch/arc/kernel/jump_label.c
> @@ -0,0 +1,170 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/kernel.h>
> +#include <linux/jump_label.h>
> +
> +#include "asm/cacheflush.h"
> +
> +#define JUMPLABEL_ERR	"ARC: jump_label: ERROR: "
> +
> +/* Halt system on fatal error to make debug easier */
> +#define arc_jl_fatal(format...)						\
> +({									\
> +	pr_err(JUMPLABEL_ERR format);					\
> +	BUG();								\
> +})
> +
> +static inline u32 arc_gen_nop(void)
> +{
> +	/* 1x 32bit NOP in middle endian */
> +	return 0x7000264a;
> +}
> +
> +/*
> + * Atomic update of patched instruction is only available if this
> + * instruction doesn't cross L1 cache line boundary. You can read about
> + * the way we achieve this in arc/include/asm/jump_label.h
> + */
> +static inline void instruction_align_assert(void *addr, int len)
> +{
> +	unsigned long a = (unsigned long)addr;
> +
> +	if ((a >> L1_CACHE_SHIFT) != ((a + len - 1) >> L1_CACHE_SHIFT))
> +		arc_jl_fatal("instruction (addr %px) cross L1 cache line border",
> +			     addr);
> +}
> +
> +/*
> + * ARCv2 'Branch unconditionally' instruction:
> + * 00000ssssssssss1SSSSSSSSSSNRtttt
> + * s S[n:0] lower bits signed immediate (number is bitfield size)
> + * S S[m:n+1] upper bits signed immediate (number is bitfield size)
> + * t S[24:21] upper bits signed immediate (branch unconditionally far)
> + * N N <.d> delay slot mode
> + * R R Reserved
> + */
> +static inline u32 arc_gen_branch(jump_label_t pc, jump_label_t target)
> +{
> +	u32 instruction_l, instruction_r;
> +	u32 pcl = pc & GENMASK(31, 2);
> +	u32 u_offset = target - pcl;
> +	u32 s, S, t;
> +
> +	/*
> +	 * Offset in 32-bit branch instruction must to fit into s25.
> +	 * Something is terribly broken if we get such huge offset within one
> +	 * function.
> +	 */
> +	if ((s32)u_offset < -16777216 || (s32)u_offset > 16777214)
> +		arc_jl_fatal("gen branch with offset (%d) not fit in s25",
> +			     (s32)u_offset);
> +
> +	/*
> +	 * All instructions are aligned by 2 bytes so we should never get offset
> +	 * here which is not 2 bytes aligned.
> +	 */
> +	if (u_offset & 0x1)
> +		arc_jl_fatal("gen branch with offset (%d) unaligned to 2 bytes",
> +			     (s32)u_offset);
> +
> +	s = (u_offset >> 1)  & GENMASK(9, 0);
> +	S = (u_offset >> 11) & GENMASK(9, 0);
> +	t = (u_offset >> 21) & GENMASK(3, 0);
> +
> +	/* 00000ssssssssss1 */
> +	instruction_l = (s << 1) | 0x1;
> +	/* SSSSSSSSSSNRtttt */
> +	instruction_r = (S << 6) | t;
> +
> +	return (instruction_r << 16) | (instruction_l & GENMASK(15, 0));
> +}
> +
> +void arch_jump_label_transform(struct jump_entry *entry,
> +			       enum jump_label_type type)
> +{
> +	jump_label_t *instr_addr = (jump_label_t *)entry->code;
> +	u32 instr;
> +
> +	instruction_align_assert(instr_addr, JUMP_LABEL_NOP_SIZE);
> +
> +	if (type == JUMP_LABEL_JMP)
> +		instr = arc_gen_branch(entry->code, entry->target);
> +	else
> +		instr = arc_gen_nop();
> +
> +	WRITE_ONCE(*instr_addr, instr);
> +	flush_icache_range(entry->code, entry->code + JUMP_LABEL_NOP_SIZE);
> +}
> +
> +void arch_jump_label_transform_static(struct jump_entry *entry,
> +				      enum jump_label_type type)
> +{
> +	/*
> +	 * We use only one NOP type (1x, 4 byte) in arch_static_branch, so
> +	 * there's no need to patch an identical NOP over the top of it here.
> +	 * The generic code calls 'arch_jump_label_transform' if the NOP needs
> +	 * to be replaced by a branch, so 'arch_jump_label_transform_static' is
> +	 * never called with type other than JUMP_LABEL_NOP.
> +	 */
> +	BUG_ON(type != JUMP_LABEL_NOP);
> +}
> +
> +#ifdef CONFIG_ARC_DBG_JUMP_LABEL
> +#define SELFTEST_MSG	"ARC: instruction generation self-test: "
> +
> +struct arc_gen_branch_testdata {
> +	jump_label_t pc;
> +	jump_label_t target_address;
> +	u32 expected_instr;
> +};
> +
> +static __init int branch_gen_test(const struct arc_gen_branch_testdata *test)
> +{
> +	u32 instr_got;
> +
> +	instr_got = arc_gen_branch(test->pc, test->target_address);
> +	if (instr_got == test->expected_instr)
> +		return 0;
> +
> +	pr_err(SELFTEST_MSG "FAIL:\n arc_gen_branch(0x%08x, 0x%08x) != 0x%08x, got 0x%08x\n",
> +	       test->pc, test->target_address,
> +	       test->expected_instr, instr_got);
> +
> +	return -EFAULT;
> +}
> +
> +/*
> + * Offset field in branch instruction is not continuous. Test all
> + * available offset field and sign combinations. Test data is generated
> + * from real working code.
> + */
> +static const struct arc_gen_branch_testdata arcgenbr_test_data[] __initconst = {
> +	{0x90007548, 0x90007514, 0xffcf07cd}, /* tiny (-52) offs */
> +	{0x9000c9c0, 0x9000c782, 0xffcf05c3}, /* tiny (-574) offs */
> +	{0x9000cc1c, 0x9000c782, 0xffcf0367}, /* tiny (-1178) offs */
> +	{0x9009dce0, 0x9009d106, 0xff8f0427}, /* small (-3034) offs */
> +	{0x9000f5de, 0x90007d30, 0xfc0f0755}, /* big  (-30892) offs */
> +	{0x900a2444, 0x90035f64, 0xc9cf0321}, /* huge (-443616) offs */
> +	{0x90007514, 0x9000752c, 0x00000019}, /* tiny (+24) offs */
> +	{0x9001a578, 0x9001a77a, 0x00000203}, /* tiny (+514) offs */
> +	{0x90031ed8, 0x90032634, 0x0000075d}, /* tiny (+1884) offs */
> +	{0x9008c7f2, 0x9008d3f0, 0x00400401}, /* small (+3072) offs */
> +	{0x9000bb38, 0x9003b340, 0x17c00009}, /* big  (+194568) offs */
> +	{0x90008f44, 0x90578d80, 0xb7c2063d}  /* huge (+5701180) offs */
> +};
> +
> +static __init int instr_gen_test(void)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(arcgenbr_test_data); i++)
> +		if (branch_gen_test(&arcgenbr_test_data[i]))
> +			return -EFAULT;
> +
> +	pr_info(SELFTEST_MSG "OK\n");
> +
> +	return 0;
> +}
> +early_initcall(instr_gen_test);
> +
> +#endif /* CONFIG_ARC_DBG_JUMP_LABEL */