On Tue, Nov 15, 2022 at 11:27:03AM +0000, Will Deacon wrote:

On Thu, Nov 03, 2022 at 05:05:20PM +0000, Mark Rutland wrote:

quoted

This commit replaces arm64's support for FTRACE_WITH_REGS with support
for FTRACE_WITH_ARGS. This removes some overhead and complexity, and
removes some latent issues with inconsistent presentation of struct
pt_regs (which can only be reliably saved/restored at exception
boundaries).

[...]

quoted

@@ -78,10 +77,71 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
 	return addr;
 }
 
-#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
 struct dyn_ftrace;
 struct ftrace_ops;
-struct ftrace_regs;
+
+#define arch_ftrace_get_regs(regs) NULL
+
+struct ftrace_regs {
+	/* x0 - x8 */
+	unsigned long regs[9];
+	unsigned long __unused;
+
+	unsigned long fp;
+	unsigned long lr;
+
+	unsigned long sp;
+	unsigned long pc;
+};
+
+static __always_inline unsigned long
+ftrace_regs_get_instruction_pointer(const struct ftrace_regs *fregs)
+{
+	return fregs->pc;
+}
+
+static __always_inline void
+ftrace_regs_set_instruction_pointer(struct ftrace_regs *fregs,
+				    unsigned long pc)
+{
+	fregs->pc = pc;
+}
+
+static __always_inline unsigned long
+ftrace_regs_get_stack_pointer(const struct ftrace_regs *fregs)
+{
+	return fregs->sp;
+}
+
+static __always_inline unsigned long
+ftrace_regs_get_argument(struct ftrace_regs *fregs, unsigned int n)
+{
+	if (n < 8)
+		return fregs->regs[n];

Where does this '8' come from?

Because in AAPCS64 the arguments are in x0 to x7, as mentioned in the commit
message:

| Per AAPCS64, all function call argument and return values are held in
| the following GPRs:
| 
| * X0 - X7 : parameter / result registers
| * X8      : indirect result location register
| * SP      : stack pointer (AKA SP)

The 'indirect result location register' would be used when returning large
structures, and isn't a function argument as such.

The logic is the same as in regs_get_kernel_argument() for pt_regs.

I can add a comment here to explain that, if that would help?

The rest of the registers are as described in the commit message (and I now
spot a typo that I'll go fix):

| Additionally, ad function call boundaries, the following GPRs hold
| context/return information:
| 
| * X29 : frame pointer (AKA FP)
| * X30 : link register (AKA LR)
| 
| ... and for ftrace we need to capture the instrumented address:
| 
|  * PC  : program counter
| 
| No other GPRs are relevant, as none of the other arguments hold
| parameters or return values:
| 
| * X9  - X17 : temporaries, may be clobbered
| * X18       : shadow call stack pointer (or temorary)
| * X19 - X28 : callee saved

Thanks,
Mark.

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