if a driver needs cpu_xcall, it can depend on xcall and enable
compilation of the xcall code and this glue.

ok?

Index: amd64/cpu.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/cpu.c,v
diff -u -p -r1.196 cpu.c
--- amd64/cpu.c	5 Jun 2025 09:29:54 -0000	1.196
+++ amd64/cpu.c	13 Jul 2025 05:50:03 -0000
@@ -637,6 +637,10 @@ cpu_attach(struct device *parent, struct
 #endif
 
 #if defined(MULTIPROCESSOR)
+#if NXCALL > 0
+	cpu_xcall_establish(ci);
+#endif
+
 	/*
 	 * Allocate UPAGES contiguous pages for the idle PCB and stack.
 	 */
Index: amd64/ipifuncs.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/ipifuncs.c,v
diff -u -p -r1.39 ipifuncs.c
--- amd64/ipifuncs.c	7 Jun 2024 16:53:35 -0000	1.39
+++ amd64/ipifuncs.c	13 Jul 2025 05:50:03 -0000
@@ -108,6 +108,11 @@ void (*ipifunc[X86_NIPI])(struct cpu_inf
 	NULL,
 #endif
 	x86_64_ipi_wbinvd,
+#if NXCALL > 0
+	cpu_xcall_dispatch,
+#else
+	NULL,
+#endif
 };
 
 void
Index: include/cpu.h
===================================================================
RCS file: /cvs/src/sys/arch/amd64/include/cpu.h,v
diff -u -p -r1.180 cpu.h
--- include/cpu.h	28 Apr 2025 16:18:25 -0000	1.180
+++ include/cpu.h	13 Jul 2025 05:50:04 -0000
@@ -42,9 +42,12 @@
  * Definitions unique to x86-64 cpu support.
  */
 #ifdef _KERNEL
+#include "xcall.h"
+
 #include <machine/frame.h>
 #include <machine/segments.h>		/* USERMODE */
 #include <machine/intrdefs.h>
+#include <sys/xcall.h>
 #endif /* _KERNEL */
 
 #include <sys/clockintr.h>
@@ -215,6 +218,9 @@ struct cpu_info {
 
 #ifdef MULTIPROCESSOR
 	struct srp_hazard	ci_srp_hazards[SRP_HAZARD_NUM];
+#if NXCALL > 0
+	struct xcall_cpu	ci_xcall;
+#endif
 #define __HAVE_UVM_PERCPU
 	struct uvm_pmr_cache	ci_uvm;		/* [o] page cache */
 #endif
Index: include/intr.h
===================================================================
RCS file: /cvs/src/sys/arch/amd64/include/intr.h,v
diff -u -p -r1.36 intr.h
--- include/intr.h	11 Jun 2025 09:57:01 -0000	1.36
+++ include/intr.h	13 Jul 2025 05:50:04 -0000
@@ -222,7 +222,9 @@ void x86_ipi_handler(void);
 void x86_setperf_ipi(struct cpu_info *);
 
 extern void (*ipifunc[X86_NIPI])(struct cpu_info *);
-#endif
+
+#define cpu_xcall_ipi(_ci) x86_send_ipi((_ci), X86_IPI_XCALL)
+#endif /* MULTIPROCESSOR */
 
 #endif /* !_LOCORE */
 
Index: include/intrdefs.h
===================================================================
RCS file: /cvs/src/sys/arch/amd64/include/intrdefs.h,v
diff -u -p -r1.24 intrdefs.h
--- include/intrdefs.h	26 May 2024 13:37:31 -0000	1.24
+++ include/intrdefs.h	13 Jul 2025 05:50:04 -0000
@@ -85,8 +85,9 @@
 #define X86_IPI_START_VMM		0x00000100
 #define X86_IPI_STOP_VMM		0x00000200
 #define X86_IPI_WBINVD			0x00000400
+#define X86_IPI_XCALL			0x00000800
 
-#define X86_NIPI			12
+#define X86_NIPI			13
 
 #define IREENT_MAGIC	0x18041969