commit 3715bf9b86a86ed6a3a857cabfc7dff5d70b409b
Author: Russ Cox <rsc@golang.org>
Date:   Sat Mar 1 16:47:07 2025 -0500

    sys/src/9: allow floating point in note handlers
    
    It's well past time to allow floating point in note handlers,
    since that also allows SSE and other SIMD code.
    
    mk fpnote.run in /usr/glenda/src for a test.
    
    For golang/go#62507.

--- a/sys/src/9/pc/dat.h
+++ b/sys/src/9/pc/dat.h
@@ -57,6 +57,10 @@ struct Label
 
 /*
  * FPsave.status
+ *
+ * FPinit: never used
+ * FPactive: in use, registers are live
+ * FPinactive: not in use, registers are saved
  */
 enum
 {
@@ -64,9 +68,11 @@ enum
 	FPinit=		0,
 	FPactive=	1,
 	FPinactive=	2,
+	FPnotestart = 3,
 
-	/* the following is a bit that can be or'd into the state */
-	FPillegal=	0x100,
+	/* state repeated 3 bits up for note handler */
+	FPnoteshift = 3,
+	FPnotemask = 7<<3,
 };
 
 struct	FPstate			/* x87 fpu state */
--- a/sys/src/9/pc/main.c
+++ b/sys/src/9/pc/main.c
@@ -277,7 +277,7 @@ userinit(void)
 	 * Kernel Stack
 	 *
 	 * N.B. make sure there's enough space for syscall to check
-	 *	for valid args and 
+	 *	for valid args and
 	 *	4 bytes for gotolabel's return PC
 	 */
 	p->sched.pc = (ulong)init0;
@@ -640,10 +640,34 @@ mathemu(Ureg *ureg, void*)
 {
 	ulong status, control;
 
-	if(up->fpstate & FPillegal){
-		/* someone did floating point in a note handler */
-		postnote(up, 1, "sys: floating point in note handler", NDebug);
-		return;
+ 	if(up->fpstate & FPnotemask){
+ 		/* in note handler */
+ 		switch(up->fpstate>>FPnoteshift){
+ 		case FPnotestart:
+ 			/* no FP used yet; copy state at time of note */
+ 			if((up->fpstate&~FPnotemask) == FPinit) {
+ 				/* no FP in the process yet */
+ 				up->fpstate = (up->fpstate&~FPnotemask) | (FPactive<<FPnoteshift);
+ 				fpinit();
+ 				return;
+ 			}
+ 			up->notefpsave = up->fpsave;
+ 			/* fall through */
+ 		case FPinactive:
+ 			/* restore state */
+ 			mathstate(&status, nil, &control);
+ 			if((status & ~control) & 0x07F)
+ 				break;
+ 			fprestore(&up->notefpsave);
+ 			up->fpstate = (up->fpstate&~FPnotemask) | (FPactive<<FPnoteshift);
+ 			return;
+
+ 		case FPactive:
+ 			panic("note math emu pid %ld %s pc %#lux",
+ 				up->pid, up->text, ureg->pc);
+ 		}
+ 		postnote(up, 1, "sys: floating point exception in note handler", NDebug);
+ 		return;
 	}
 	switch(up->fpstate){
 	case FPinit:
@@ -723,7 +747,15 @@ procsave(Proc *p)
 
 	cycles(&t);
 	p->pcycles += t;
-	if(p->fpstate == FPactive){
+ 	if(p->fpstate&FPnotemask){
+ 		if((p->fpstate>>FPnoteshift) == FPactive){
+ 			if(p->state == Moribund)
+ 				fpclear();
+ 			else
+ 				fpsave(&p->notefpsave);
+ 			p->fpstate = (p->fpstate&~FPnotemask) | (FPinactive<<FPnoteshift);
+ 		}
+ 	} else if(p->fpstate == FPactive){
 		if(p->state == Moribund)
 			fpclear();
 		else{
--- a/sys/src/9/pc/trap.c
+++ b/sys/src/9/pc/trap.c
@@ -699,9 +699,14 @@ syscall(Ureg* ureg)
 		startns = todget(nil);
 	}
 
-	if(scallnr == RFORK && up->fpstate == FPactive){
-		fpsave(&up->fpsave);
-		up->fpstate = FPinactive;
+ 	if(scallnr == RFORK){
+ 		if(up->fpstate == FPactive){
+ 			fpsave(&up->fpsave);
+ 			up->fpstate = FPinactive;
+ 		}else if((up->fpstate>>FPnoteshift) == FPactive){
+ 			fpsave(&up->notefpsave);
+ 			up->fpstate = (up->fpstate&~FPnotemask) | (FPinactive<<FPnoteshift);
+ 		}
 	}
 	spllo();
 
@@ -795,7 +800,7 @@ notify(Ureg* ureg)
 		fpsave(&up->fpsave);
 		up->fpstate = FPinactive;
 	}
-	up->fpstate |= FPillegal;
+ 	up->fpstate |= FPnotestart<<FPnoteshift;
 
 	s = spllo();
 	qlock(&up->debug);
@@ -879,7 +884,9 @@ noted(Ureg* ureg, ulong arg0)
 
 	nureg = up->ureg;	/* pointer to user returned Ureg struct */
 
-	up->fpstate &= ~FPillegal;
+	if((up->fpstate>>FPnoteshift) == FPactive)
+		fpoff();
+	up->fpstate &= ~FPnotemask;
 
 	/* sanity clause */
 	oureg = (ulong)nureg;
--- a/sys/src/9/port/portdat.h
+++ b/sys/src/9/port/portdat.h
@@ -753,6 +753,7 @@ struct Proc
 	short	notified;	/* sysnoted is due */
 	Note	lastnote;
 	int	(*notify)(void*, char*);
+	FPsave	notefpsave;
 
 	Lock	*lockwait;
 	Lock	*lastlock;	/* debugging */
--- a/sys/src/9/port/sysproc.c
+++ b/sys/src/9/port/sysproc.c
@@ -181,7 +181,10 @@ sysrfork(ulong *arg)
 		p->noteid = up->noteid;
 
 	/* don't penalize the child, it hasn't done FP in a note handler. */
-	p->fpstate = up->fpstate & ~FPillegal;
+	if((up->fpstate>>FPnoteshift) != 0){
+		fpoff();
+		p->fpstate &= ~FPnotemask;
+	}
 	pid = p->pid;
 	memset(p->time, 0, sizeof(p->time));
 	p->time[TReal] = MACHP(0)->ticks;