runtime: unify asmcgocall and systemstack traceback setup

Both asmcgocall and systemstack need to save the calling Go code's
context for use by traceback, but they do it differently.
Systemstack's appraoch is better, because it doesn't require a
special case in traceback.
So make them both use that.

While we are here, the fake mstart caller in systemstack is
no longer needed and can be removed.
(traceback knows to stop in systemstack because of the writes to SP.)

Also remove the fake mstarts in sys_windows_*.s.

And while we are there, fix the control flow guard code in sys_windows_arm.s.
The current code is using pointers to a stack frame that technically is gone
once we hit the RET instruction. Clearly it's working OK, but better not to depend
on data below SP being preserved, even for just a few instructions.
Store the value we need in other registers instead.
(This code is only used for pushing a sigpanic call, which does not
actually return to the site of the fault and therefore doesn't need to
preserve any of the registers.)

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: Id1e3ef5e54f7ad786e4b87043f2626eba7c3bbd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/288799
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 12 insertions, 14 deletions

diff --git a/src/runtime/asm_riscv64.s b/src/runtime/asm_riscv64.s
index 3d0349471a..fb7c6530dc 100644
--- a/src/runtime/asm_riscv64.s
+++ b/src/runtime/asm_riscv64.s
@@ -114,21 +114,12 @@ TEXT runtime·systemstack(SB), NOSPLIT, $0-8
 switch:
 	// save our state in g->sched. Pretend to
 	// be systemstack_switch if the G stack is scanned.
-	MOV	$runtime·systemstack_switch(SB), T2
-	ADD	$8, T2	// get past prologue
-	MOV	T2, (g_sched+gobuf_pc)(g)
-	MOV	X2, (g_sched+gobuf_sp)(g)
-	MOV	ZERO, (g_sched+gobuf_lr)(g)
-	MOV	g, (g_sched+gobuf_g)(g)
+	CALL	gosave_systemstack_switch<>(SB)
 
 	// switch to g0
 	MOV	T1, g
 	CALL	runtime·save_g(SB)
 	MOV	(g_sched+gobuf_sp)(g), T0
-	// make it look like mstart called systemstack on g0, to stop traceback
-	ADD	$-8, T0
-	MOV	$runtime·mstart(SB), T1
-	MOV	T1, 0(T0)
 	MOV	T0, X2
 
 	// call target function
@@ -297,12 +288,19 @@ TEXT runtime·mcall(SB), NOSPLIT|NOFRAME, $0-8
 	JALR	RA, T1
 	JMP	runtime·badmcall2(SB)
 
-// Save state of caller into g->sched. Smashes X31.
-TEXT gosave<>(SB),NOSPLIT|NOFRAME,$0
-	MOV	X1, (g_sched+gobuf_pc)(g)
+// Save state of caller into g->sched,
+// but using fake PC from systemstack_switch.
+// Must only be called from functions with no locals ($0)
+// or else unwinding from systemstack_switch is incorrect.
+// Smashes X31.
+TEXT gosave_systemstack_switch<>(SB),NOSPLIT|NOFRAME,$0
+	MOV	$runtime·systemstack_switch(SB), X31
+	ADD	$8, X31	// get past prologue
+	MOV	X31, (g_sched+gobuf_pc)(g)
 	MOV	X2, (g_sched+gobuf_sp)(g)
 	MOV	ZERO, (g_sched+gobuf_lr)(g)
 	MOV	ZERO, (g_sched+gobuf_ret)(g)
+	MOV	g, (g_sched+gobuf_g)(g)
 	// Assert ctxt is zero. See func save.
 	MOV	(g_sched+gobuf_ctxt)(g), X31
 	BEQ	ZERO, X31, 2(PC)
@@ -327,7 +325,7 @@ TEXT ·asmcgocall(SB),NOSPLIT,$0-20
 	MOV	m_g0(X6), X7
 	BEQ	X7, g, g0
 
-	CALL	gosave<>(SB)
+	CALL	gosave_systemstack_switch<>(SB)
 	MOV	X7, g
 	CALL	runtime·save_g(SB)
 	MOV	(g_sched+gobuf_sp)(g), X2