s390/ftrace: implement hotpatching

s390 allows hotpatching the mask of a conditional jump instruction.
Make use of this feature in order to avoid the expensive stop_machine()
call.

The new trampolines are split in 3 stages:

- A first stage is a 6-byte relative conditional long branch located at
  each function's entry point. Its offset always points to the second
  stage for the corresponding function, and its mask is either all 0s
  (ftrace off) or all 1s (ftrace on). The code for flipping the mask is
  borrowed from ftrace_{enable,disable}_ftrace_graph_caller. After
  flipping, ftrace_arch_code_modify_post_process() syncs with all the
  other CPUs by sending SIGPs.

- Second stages for vmlinux are stored in a separate part of the .text
  section reserved by the linker script, and in dynamically allocated
  memory for modules. This prevents the icache pollution. The total
  size of second stages is about 1.5% of that of the kernel image.

  Putting second stages in the .bss section is possible and decreases
  the size of the non-compressed vmlinux, but splits the kernel 1:1
  mapping, which is a bad tradeoff.

  Each second stage contains a call to the third stage, a pointer to
  the part of the intercepted function right after the first stage, and
  a pointer to an interceptor function (e.g. ftrace_caller).

  Second stages are 8-byte aligned for the future direct calls
  implementation.

- There are only two copies of the third stage: in the .text section
  for vmlinux and in dynamically allocated memory for modules. It can be
  an expoline, which is relatively large, so inlining it into each
  second stage is prohibitively expensive.

As a result of this organization, phoronix-test-suite with ftrace off
does not show any performance degradation.

Suggested-by: Sven Schnelle <svens@linux.ibm.com>
Suggested-by: Vasily Gorbik <gor@linux.ibm.com>
Co-developed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20210728212546.128248-3-iii@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/ftrace.h

@@ -18,7 +18,6 @@
 void ftrace_caller(void);
 
 extern char ftrace_graph_caller_end;
-extern unsigned long ftrace_plt;
 extern void *ftrace_func;
 
 struct dyn_arch_ftrace { };
@@ -31,53 +30,18 @@ struct dyn_arch_ftrace { };
 
 struct module;
 struct dyn_ftrace;
-/*
- * Either -mhotpatch or -mnop-mcount is used - no explicit init is required
- */
-static inline int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec) { return 0; }
+
+bool ftrace_need_init_nop(void);
+#define ftrace_need_init_nop ftrace_need_init_nop
+
+int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
 #define ftrace_init_nop ftrace_init_nop
 
 static inline unsigned long ftrace_call_adjust(unsigned long addr)
 {
 	return addr;
 }
 
-struct ftrace_insn {
-	u16 opc;
-	s32 disp;
-} __packed;
-
-static inline void ftrace_generate_nop_insn(struct ftrace_insn *insn)
-{
-#ifdef CONFIG_FUNCTION_TRACER
-	/* brcl 0,0 */
-	insn->opc = 0xc004;
-	insn->disp = 0;
-#endif
-}
-
-static inline int is_ftrace_nop(struct ftrace_insn *insn)
-{
-#ifdef CONFIG_FUNCTION_TRACER
-	if (insn->disp == 0)
-		return 1;
-#endif
-	return 0;
-}
-
-static inline void ftrace_generate_call_insn(struct ftrace_insn *insn,
-					     unsigned long ip)
-{
-#ifdef CONFIG_FUNCTION_TRACER
-	unsigned long target;
-
-	/* brasl r0,ftrace_caller */
-	target = is_module_addr((void *) ip) ? ftrace_plt : FTRACE_ADDR;
-	insn->opc = 0xc005;
-	insn->disp = (target - ip) / 2;
-#endif
-}
-
 /*
  * Even though the system call numbers are identical for s390/s390x a
  * different system call table is used for compat tasks. This may lead

arch/s390/include/asm/ftrace.lds.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef DIV_ROUND_UP
+#define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
+#endif
+
+#define SIZEOF_MCOUNT_LOC_ENTRY 8
+#define SIZEOF_FTRACE_HOTPATCH_TRAMPOLINE 24
+#define FTRACE_HOTPATCH_TRAMPOLINES_SIZE(n)				       \
+	DIV_ROUND_UP(SIZEOF_FTRACE_HOTPATCH_TRAMPOLINE * (n),		       \
+		     SIZEOF_MCOUNT_LOC_ENTRY)
+
+#ifdef CONFIG_FUNCTION_TRACER
+#define FTRACE_HOTPATCH_TRAMPOLINES_TEXT				       \
+	. = ALIGN(8);							       \
+	__ftrace_hotpatch_trampolines_start = .;			       \
+	. = . + FTRACE_HOTPATCH_TRAMPOLINES_SIZE(__stop_mcount_loc -	       \
+						 __start_mcount_loc);	       \
+	__ftrace_hotpatch_trampolines_end = .;
+#else
+#define FTRACE_HOTPATCH_TRAMPOLINES_TEXT
+#endif

arch/s390/include/asm/module.h

@@ -28,6 +28,14 @@ struct mod_arch_specific {
 	int nsyms;
 	/* Additional symbol information (got and plt offsets). */
 	struct mod_arch_syminfo *syminfo;
+#ifdef CONFIG_FUNCTION_TRACER
+	/* Start of memory reserved for ftrace hotpatch trampolines. */
+	struct ftrace_hotpatch_trampoline *trampolines_start;
+	/* End of memory reserved for ftrace hotpatch trampolines. */
+	struct ftrace_hotpatch_trampoline *trampolines_end;
+	/* Next unused ftrace hotpatch trampoline slot. */
+	struct ftrace_hotpatch_trampoline *next_trampoline;
+#endif /* CONFIG_FUNCTION_TRACER */
 };
 
 #endif /* _ASM_S390_MODULE_H */

arch/s390/kernel/ftrace.c

@@ -18,8 +18,11 @@
 #include <trace/syscall.h>
 #include <asm/asm-offsets.h>
 #include <asm/cacheflush.h>
+#include <asm/ftrace.lds.h>
+#include <asm/nospec-branch.h>
 #include <asm/set_memory.h>
 #include "entry.h"
+#include "ftrace.h"
 
 /*
  * To generate function prologue either gcc's hotpatch feature (since gcc 4.8)
@@ -41,19 +44,176 @@
  */
 
 void *ftrace_func __read_mostly = ftrace_stub;
-unsigned long ftrace_plt;
+struct ftrace_insn {
+	u16 opc;
+	s32 disp;
+} __packed;
+
+asm(
+	"	.align 16\n"
+	"ftrace_shared_hotpatch_trampoline_br:\n"
+	"	lmg	%r0,%r1,2(%r1)\n"
+	"	br	%r1\n"
+	"ftrace_shared_hotpatch_trampoline_br_end:\n"
+);
+
+#ifdef CONFIG_EXPOLINE
+asm(
+	"	.align 16\n"
+	"ftrace_shared_hotpatch_trampoline_ex:\n"
+	"	lmg	%r0,%r1,2(%r1)\n"
+	"	ex	%r0," __stringify(__LC_BR_R1) "(%r0)\n"
+	"	j	.\n"
+	"ftrace_shared_hotpatch_trampoline_ex_end:\n"
+);
+
+asm(
+	"	.align 16\n"
+	"ftrace_shared_hotpatch_trampoline_exrl:\n"
+	"	lmg	%r0,%r1,2(%r1)\n"
+	"	.insn	ril,0xc60000000000,%r0,0f\n" /* exrl */
+	"	j	.\n"
+	"0:	br	%r1\n"
+	"ftrace_shared_hotpatch_trampoline_exrl_end:\n"
+);
+#endif /* CONFIG_EXPOLINE */
+
+#ifdef CONFIG_MODULES
+static char *ftrace_plt;
+
+asm(
+	"	.data\n"
+	"ftrace_plt_template:\n"
+	"	basr	%r1,%r0\n"
+	"	lg	%r1,0f-.(%r1)\n"
+	"	br	%r1\n"
+	"0:	.quad	ftrace_caller\n"
+	"ftrace_plt_template_end:\n"
+	"	.previous\n"
+);
+#endif /* CONFIG_MODULES */
+
+static const char *ftrace_shared_hotpatch_trampoline(const char **end)
+{
+	const char *tstart, *tend;
+
+	tstart = ftrace_shared_hotpatch_trampoline_br;
+	tend = ftrace_shared_hotpatch_trampoline_br_end;
+#ifdef CONFIG_EXPOLINE
+	if (!nospec_disable) {
+		tstart = ftrace_shared_hotpatch_trampoline_ex;
+		tend = ftrace_shared_hotpatch_trampoline_ex_end;
+		if (test_facility(35)) { /* exrl */
+			tstart = ftrace_shared_hotpatch_trampoline_exrl;
+			tend = ftrace_shared_hotpatch_trampoline_exrl_end;
+		}
+	}
+#endif /* CONFIG_EXPOLINE */
+	if (end)
+		*end = tend;
+	return tstart;
+}
+
+bool ftrace_need_init_nop(void)
+{
+	return ftrace_shared_hotpatch_trampoline(NULL);
+}
+
+int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
+{
+	static struct ftrace_hotpatch_trampoline *next_vmlinux_trampoline =
+		__ftrace_hotpatch_trampolines_start;
+	static const char orig[6] = { 0xc0, 0x04, 0x00, 0x00, 0x00, 0x00 };
+	static struct ftrace_hotpatch_trampoline *trampoline;
+	struct ftrace_hotpatch_trampoline **next_trampoline;
+	struct ftrace_hotpatch_trampoline *trampolines_end;
+	struct ftrace_hotpatch_trampoline tmp;
+	struct ftrace_insn *insn;
+	const char *shared;
+	s32 disp;
+
+	BUILD_BUG_ON(sizeof(struct ftrace_hotpatch_trampoline) !=
+		     SIZEOF_FTRACE_HOTPATCH_TRAMPOLINE);
+
+	next_trampoline = &next_vmlinux_trampoline;
+	trampolines_end = __ftrace_hotpatch_trampolines_end;
+	shared = ftrace_shared_hotpatch_trampoline(NULL);
+#ifdef CONFIG_MODULES
+	if (mod) {
+		next_trampoline = &mod->arch.next_trampoline;
+		trampolines_end = mod->arch.trampolines_end;
+		shared = ftrace_plt;
+	}
+#endif
+
+	if (WARN_ON_ONCE(*next_trampoline >= trampolines_end))
+		return -ENOMEM;
+	trampoline = (*next_trampoline)++;
+
+	/* Check for the compiler-generated fentry nop (brcl 0, .). */
+	if (WARN_ON_ONCE(memcmp((const void *)rec->ip, &orig, sizeof(orig))))
+		return -EINVAL;
+
+	/* Generate the trampoline. */
+	tmp.brasl_opc = 0xc015; /* brasl %r1, shared */
+	tmp.brasl_disp = (shared - (const char *)&trampoline->brasl_opc) / 2;
+	tmp.interceptor = FTRACE_ADDR;
+	tmp.rest_of_intercepted_function = rec->ip + sizeof(struct ftrace_insn);
+	s390_kernel_write(trampoline, &tmp, sizeof(tmp));
+
+	/* Generate a jump to the trampoline. */
+	disp = ((char *)trampoline - (char *)rec->ip) / 2;
+	insn = (struct ftrace_insn *)rec->ip;
+	s390_kernel_write(&insn->disp, &disp, sizeof(disp));
+
+	return 0;
+}
 
 int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
 		       unsigned long addr)
 {
 	return 0;
 }
 
+static void ftrace_generate_nop_insn(struct ftrace_insn *insn)
+{
+	/* brcl 0,0 */
+	insn->opc = 0xc004;
+	insn->disp = 0;
+}
+
+static void ftrace_generate_call_insn(struct ftrace_insn *insn,
+				      unsigned long ip)
+{
+	unsigned long target;
+
+	/* brasl r0,ftrace_caller */
+	target = FTRACE_ADDR;
+#ifdef CONFIG_MODULES
+	if (is_module_addr((void *)ip))
+		target = (unsigned long)ftrace_plt;
+#endif /* CONFIG_MODULES */
+	insn->opc = 0xc005;
+	insn->disp = (target - ip) / 2;
+}
+
+static void brcl_disable(void *brcl)
+{
+	u8 op = 0x04; /* set mask field to zero */
+
+	s390_kernel_write((char *)brcl + 1, &op, sizeof(op));
+}
+
 int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 		    unsigned long addr)
 {
 	struct ftrace_insn orig, new, old;
 
+	if (ftrace_shared_hotpatch_trampoline(NULL)) {
+		brcl_disable((void *)rec->ip);
+		return 0;
+	}
+
 	if (copy_from_kernel_nofault(&old, (void *) rec->ip, sizeof(old)))
 		return -EFAULT;
 	/* Replace ftrace call with a nop. */
@@ -67,10 +227,22 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
 	return 0;
 }
 
+static void brcl_enable(void *brcl)
+{
+	u8 op = 0xf4; /* set mask field to all ones */
+
+	s390_kernel_write((char *)brcl + 1, &op, sizeof(op));
+}
+
 int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 {
 	struct ftrace_insn orig, new, old;
 
+	if (ftrace_shared_hotpatch_trampoline(NULL)) {
+		brcl_enable((void *)rec->ip);
+		return 0;
+	}
+
 	if (copy_from_kernel_nofault(&old, (void *) rec->ip, sizeof(old)))
 		return -EFAULT;
 	/* Replace nop with an ftrace call. */
@@ -95,22 +267,44 @@ int __init ftrace_dyn_arch_init(void)
 	return 0;
 }
 
+void arch_ftrace_update_code(int command)
+{
+	if (ftrace_shared_hotpatch_trampoline(NULL))
+		ftrace_modify_all_code(command);
+	else
+		ftrace_run_stop_machine(command);
+}
+
+static void __ftrace_sync(void *dummy)
+{
+}
+
+int ftrace_arch_code_modify_post_process(void)
+{
+	if (ftrace_shared_hotpatch_trampoline(NULL)) {
+		/* Send SIGP to the other CPUs, so they see the new code. */
+		smp_call_function(__ftrace_sync, NULL, 1);
+	}
+	return 0;
+}
+
 #ifdef CONFIG_MODULES
 
 static int __init ftrace_plt_init(void)
 {
-	unsigned int *ip;
+	const char *start, *end;
 
-	ftrace_plt = (unsigned long) module_alloc(PAGE_SIZE);
+	ftrace_plt = module_alloc(PAGE_SIZE);
 	if (!ftrace_plt)
 		panic("cannot allocate ftrace plt\n");
-	ip = (unsigned int *) ftrace_plt;
-	ip[0] = 0x0d10e310; /* basr 1,0; lg 1,10(1); br 1 */
-	ip[1] = 0x100a0004;
-	ip[2] = 0x07f10000;
-	ip[3] = FTRACE_ADDR >> 32;
-	ip[4] = FTRACE_ADDR & 0xffffffff;
-	set_memory_ro(ftrace_plt, 1);
+
+	start = ftrace_shared_hotpatch_trampoline(&end);
+	if (!start) {
+		start = ftrace_plt_template;
+		end = ftrace_plt_template_end;
+	}
+	memcpy(ftrace_plt, start, end - start);
+	set_memory_ro((unsigned long)ftrace_plt, 1);
 	return 0;
 }
 device_initcall(ftrace_plt_init);
@@ -147,17 +341,13 @@ NOKPROBE_SYMBOL(prepare_ftrace_return);
  */
 int ftrace_enable_ftrace_graph_caller(void)
 {
-	u8 op = 0x04; /* set mask field to zero */
-
-	s390_kernel_write(__va(ftrace_graph_caller)+1, &op, sizeof(op));
+	brcl_disable(__va(ftrace_graph_caller));
 	return 0;
 }
 
 int ftrace_disable_ftrace_graph_caller(void)
 {
-	u8 op = 0xf4; /* set mask field to all ones */
-
-	s390_kernel_write(__va(ftrace_graph_caller)+1, &op, sizeof(op));
+	brcl_enable(__va(ftrace_graph_caller));
 	return 0;
 }