4 files changed, 24 insertions, 5 deletions
diff --git a/src/runtime/mkpreempt.go b/src/runtime/mkpreempt.go
index 29e8288129..2bd2ef07fa 100644
--- a/src/runtime/mkpreempt.go
+++ b/src/runtime/mkpreempt.go
@@ -160,7 +160,7 @@ func writeXRegs(arch string, l *layout) {
 	fmt.Fprintf(g.w, `
 package runtime
 
-type xRegState struct {
+type xRegs struct {
 `)
 	pos := 0
 	for _, reg := range l.regs {
diff --git a/src/runtime/preempt.go b/src/runtime/preempt.go
index d053747d3a..22727df74e 100644
--- a/src/runtime/preempt.go
+++ b/src/runtime/preempt.go
@@ -302,7 +302,16 @@ func canPreemptM(mp *m) bool {
 // asyncPreempt is implemented in assembly.
 func asyncPreempt()
 
+// asyncPreempt2 is the Go continuation of asyncPreempt.
+//
+// It must be deeply nosplit because there's untyped data on the stack from
+// asyncPreempt.
+//
+// It must not have any write barriers because we need to limit the amount of
+// stack it uses.
+//
 //go:nosplit
+//go:nowritebarrierrec
 func asyncPreempt2() {
 	// We can't grow the stack with untyped data from asyncPreempt, so switch to
 	// the system stack right away.
diff --git a/src/runtime/preempt_amd64.go b/src/runtime/preempt_amd64.go
index 44838a1df2..88c0ddd34a 100644
--- a/src/runtime/preempt_amd64.go
+++ b/src/runtime/preempt_amd64.go
@@ -2,7 +2,7 @@
 
 package runtime
 
-type xRegState struct {
+type xRegs struct {
 	Z0  [64]byte
 	Z1  [64]byte
 	Z2  [64]byte
diff --git a/src/runtime/preempt_xreg.go b/src/runtime/preempt_xreg.go
index f0a47c15d9..9e05455ddb 100644
--- a/src/runtime/preempt_xreg.go
+++ b/src/runtime/preempt_xreg.go
@@ -19,7 +19,17 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"internal/runtime/sys"
+	"unsafe"
+)
+
+// xRegState is long-lived extended register state. It is allocated off-heap and
+// manually managed.
+type xRegState struct {
+	_    sys.NotInHeap // Allocated from xRegAlloc
+	regs xRegs
+}
 
 // xRegPerG stores extended register state while a goroutine is asynchronously
 // preempted. This is nil otherwise, so we can reuse a (likely small) pool of
@@ -31,7 +41,7 @@ type xRegPerG struct {
 type xRegPerP struct {
 	// scratch temporary per-P space where [asyncPreempt] saves the register
 	// state before entering Go. It's quickly copied to per-G state.
-	scratch xRegState
+	scratch xRegs
 
 	// cache is a 1-element allocation cache of extended register state used by
 	// asynchronous preemption. On entry to preemption, this is used as a simple
@@ -84,7 +94,7 @@ func xRegSave(gp *g) {
 	// If we ever need to save less state (e.g., avoid saving vector registers
 	// that aren't in use), we could have multiple allocation pools for
 	// different size states and copy only the registers we need.
-	*dest = pp.xRegs.scratch
+	dest.regs = pp.xRegs.scratch
 
 	// Save on the G.
 	gp.xRegs.state = dest