Index: sys/amd64/amd64/db_interface.c
===================================================================
--- sys/amd64/amd64/db_interface.c
+++ sys/amd64/amd64/db_interface.c
@@ -36,6 +36,9 @@
 #include <sys/kdb.h>
 #include <sys/pcpu.h>
 
+#include <machine/cpufunc.h>
+#include <machine/specialreg.h>
+
 #include <ddb/ddb.h>
 
 /*
@@ -62,6 +65,9 @@
 
 /*
  * Write bytes to kernel address space for debugger.
+ * We need to disable write protection temporarily so we can write
+ * things (such as break points) that might be in write-protected
+ * memory.
  */
 int
 db_write_bytes(vm_offset_t addr, size_t size, char *data)
@@ -69,15 +75,19 @@
 	jmp_buf jb;
 	void *prev_jb;
 	char *dst;
+	u_long cr0save;
 	int ret;
 
+	cr0save = rcr0();
 	prev_jb = kdb_jmpbuf(jb);
 	ret = setjmp(jb);
 	if (ret == 0) {
+		load_cr0(cr0save & ~(CR0_WP));
 		dst = (char *)addr;
 		while (size-- > 0)
 			*dst++ = *data++;
 	}
+	load_cr0(cr0save);
 	(void)kdb_jmpbuf(prev_jb);
 	return (ret);
 }
Index: sys/amd64/amd64/initcpu.c
===================================================================
--- sys/amd64/amd64/initcpu.c
+++ sys/amd64/amd64/initcpu.c
@@ -218,7 +218,7 @@
 	if (!IS_BSP() && (cpu_stdext_feature & CPUID_STDEXT_SMEP))
 		cr4 |= CR4_SMEP;
 	load_cr4(cr4);
-	if ((amd_feature & AMDID_NX) != 0) {
+	if (IS_BSP() && (amd_feature & AMDID_NX) != 0) {
 		msr = rdmsr(MSR_EFER) | EFER_NXE;
 		wrmsr(MSR_EFER, msr);
 		pg_nx = PG_NX;
Index: sys/amd64/amd64/mpboot.S
===================================================================
--- sys/amd64/amd64/mpboot.S
+++ sys/amd64/amd64/mpboot.S
@@ -221,15 +221,31 @@
 
 	/*
 	 * From here on down is executed in the kernel .text section.
-	 *
-	 * Load a real %cr3 that has all the direct map stuff and switches
-	 * off the 1GB replicated mirror.  Load a stack pointer and jump
-	 * into AP startup code in C.
 	 */
 	.text
 	.code64
 	.p2align 4,0
 entry_64:
+	/*
+	 * If the BSP reported NXE support, enable EFER.NXE for all APs
+	 * prior to loading %cr3. This avoids page faults if the AP
+	 * encounters memory marked with the NX bit prior to detecting and
+	 * enabling NXE support.
+	 */
+	movq	pg_nx, %rbx
+	testq	%rbx, %rbx
+	je	1f
+	movl	$MSR_EFER, %ecx
+	rdmsr
+	orl	$EFER_NXE, %eax
+	wrmsr
+
+1:
+	/*
+	 * Load a real %cr3 that has all the direct map stuff and switches
+	 * off the 1GB replicated mirror.  Load a stack pointer and jump
+	 * into AP startup code in C.
+	 */
 	movq	KPML4phys, %rax
 	movq	%rax, %cr3
 	movq	bootSTK, %rsp
Index: sys/amd64/amd64/pmap.c
===================================================================
--- sys/amd64/amd64/pmap.c
+++ sys/amd64/amd64/pmap.c
@@ -887,14 +887,64 @@
 	nkpt = pt_pages;
 }
 
+/*
+ * Returns the proper write/execute permission for a physical page that is
+ * part of the initial boot allocations.
+ *
+ * If the page has kernel text, it is marked as read-only. If the page has
+ * kernel read-only data, it is marked as read-only/not-executable. If the
+ * page has only read-write data, it is marked as read-write/not-executable.
+ * If the page is below/above the kernel range, it is marked as read-write.
+ *
+ * This function operates on 2M pages, since we map the kernel space that
+ * way.
+ *
+ * Note that this doesn't currently provide any protection for modules.
+ */
+static inline pt_entry_t
+bootaddr_rwx(vm_paddr_t pa)
+{
+
+	/*
+	 * Everything in the same 2M page as the start of the kernel
+	 * should be static. On the other hand, things in the same 2M
+	 * page as the end of the kernel could be read-write/executable,
+	 * as the kernel image is not guaranteed to end on a 2M boundary.
+	 */
+	if (pa < trunc_2mpage(btext - KERNBASE) ||
+	   pa >= trunc_2mpage(_end - KERNBASE))
+		return (X86_PG_RW);
+	/*
+	 * The linker should ensure that the read-only and read-write
+	 * portions don't share the same 2M page, so this shouldn't
+	 * impact read-only data. However, in any case, any page with
+	 * read-write data needs to be read-write.
+	 */
+	if (pa >= trunc_2mpage(brwsection - KERNBASE))
+		return (X86_PG_RW | pg_nx);
+	/*
+	 * Mark any 2M page containing kernel text as read-only. Mark
+	 * other pages with read-only data as read-only and not executable.
+	 * (It is likely a small portion of the read-only data section will
+	 * be marked as read-only, but executable. This should be acceptable
+	 * since the read-only protection will keep the data from changing.)
+	 * Note that fixups to the .text section will still work until we
+	 * set CR0.WP.
+	 */
+	if (pa < round_2mpage(etext - KERNBASE))
+		return (0);
+	return (pg_nx);
+}
+
 static void
 create_pagetables(vm_paddr_t *firstaddr)
 {
-	int i, j, ndm1g, nkpdpe;
+	int i, j, ndm1g, nkpdpe, nkdmpde;
 	pt_entry_t *pt_p;
 	pd_entry_t *pd_p;
 	pdp_entry_t *pdp_p;
 	pml4_entry_t *p4_p;
+	uint64_t DMPDkernphys;
 
 	/* Allocate page table pages for the direct map */
 	ndmpdp = howmany(ptoa(Maxmem), NBPDP);
@@ -913,8 +963,20 @@
 	}
 	DMPDPphys = allocpages(firstaddr, ndmpdpphys);
 	ndm1g = 0;
-	if ((amd_feature & AMDID_PAGE1GB) != 0)
+	if ((amd_feature & AMDID_PAGE1GB) != 0) {
+		/*
+		 * Calculate the number of 1G pages that will fully fit in
+		 * Maxmem.
+		 */
 		ndm1g = ptoa(Maxmem) >> PDPSHIFT;
+
+		/*
+		 * Allocate 2M pages for the kernel. These will be used in
+		 * place of the first one or more 1G pages from ndm1g.
+		 */
+		nkdmpde = howmany((vm_offset_t)(brwsection - KERNBASE), NBPDP);
+		DMPDkernphys = allocpages(firstaddr, nkdmpde);
+	}
 	if (ndm1g < ndmpdp)
 		DMPDphys = allocpages(firstaddr, ndmpdp - ndm1g);
 	dmaplimit = (vm_paddr_t)ndmpdp << PDPSHIFT;
@@ -940,11 +1002,10 @@
 	KPDphys = allocpages(firstaddr, nkpdpe);
 
 	/* Fill in the underlying page table pages */
-	/* Nominally read-only (but really R/W) from zero to physfree */
 	/* XXX not fully used, underneath 2M pages */
 	pt_p = (pt_entry_t *)KPTphys;
 	for (i = 0; ptoa(i) < *firstaddr; i++)
-		pt_p[i] = ptoa(i) | X86_PG_RW | X86_PG_V | pg_g;
+		pt_p[i] = ptoa(i) | X86_PG_V | pg_g | bootaddr_rwx(ptoa(i));
 
 	/* Now map the page tables at their location within PTmap */
 	pd_p = (pd_entry_t *)KPDphys;
@@ -954,8 +1015,8 @@
 	/* Map from zero to end of allocations under 2M pages */
 	/* This replaces some of the KPTphys entries above */
 	for (i = 0; (i << PDRSHIFT) < *firstaddr; i++)
-		pd_p[i] = (i << PDRSHIFT) | X86_PG_RW | X86_PG_V | PG_PS |
-		    pg_g;
+		pd_p[i] = (i << PDRSHIFT) | X86_PG_V | PG_PS | pg_g |
+		    bootaddr_rwx(i << PDRSHIFT);
 
 	/*
 	 * Because we map the physical blocks in 2M pages, adjust firstaddr
@@ -995,6 +1056,22 @@
 	for (j = 0; i < ndmpdp; i++, j++) {
 		pdp_p[i] = DMPDphys + ptoa(j);
 		pdp_p[i] |= X86_PG_RW | X86_PG_V | PG_U;
+	}
+
+	/*
+	 * Instead of using a 1G page for the memory containing the kernel,
+	 * use 2M pages with appropriate permissions. (If using 1G pages,
+	 * this will partially overwrite the PDPEs above.)
+	 */
+	if (ndm1g) {
+		pd_p = (pd_entry_t *)DMPDkernphys;
+		for (i = 0; i < (NPDEPG * nkdmpde); i++)
+			pd_p[i] = (i << PDRSHIFT) | X86_PG_V | PG_PS | pg_g |
+			    X86_PG_M | X86_PG_A | pg_nx |
+			    bootaddr_rwx(i << PDRSHIFT);
+		for (i = 0; i < nkdmpde; i++)
+			pdp_p[i] = (DMPDkernphys + ptoa(i)) | X86_PG_RW |
+			    X86_PG_V | PG_U;
 	}
 
 	/* And recursively map PML4 to itself in order to get PTmap */
Index: sys/amd64/include/cpu.h
===================================================================
--- sys/amd64/include/cpu.h
+++ sys/amd64/include/cpu.h
@@ -68,7 +68,9 @@
 };
 
 extern struct	cpu_ops cpu_ops;
+extern char	brwsection[];
 extern char	btext[];
+extern char	_end[];
 extern char	etext[];
 
 /* Resume hook for VMM. */
Index: sys/conf/kern.pre.mk
===================================================================
--- sys/conf/kern.pre.mk
+++ sys/conf/kern.pre.mk
@@ -120,6 +120,10 @@
 LDFLAGS+=	-Wl,--build-id=sha1
 .endif
 
+.if ${MACHINE_CPUARCH} == "amd64"
+LDFLAGS+=	-z max-page-size=2097152 -z common-page-size=4096
+.endif
+
 NORMAL_C= ${CC} -c ${CFLAGS} ${WERROR} ${PROF} ${.IMPSRC}
 NORMAL_S= ${CC:N${CCACHE_BIN}} -c ${ASM_CFLAGS} ${WERROR} ${.IMPSRC}
 PROFILE_C= ${CC} -c ${CFLAGS} ${WERROR} ${.IMPSRC}
Index: sys/conf/ldscript.amd64
===================================================================
--- sys/conf/ldscript.amd64
+++ sys/conf/ldscript.amd64
@@ -80,6 +80,7 @@
   /* Adjust the address for the data segment.  We want to adjust up to
      the same address within the page on the next page up.  */
   . = ALIGN (CONSTANT (MAXPAGESIZE)) - ((CONSTANT (MAXPAGESIZE) - .) & (CONSTANT (MAXPAGESIZE) - 1)); . = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE));
+  PROVIDE (brwsection = .);
   /* Exception handling  */
   .eh_frame       : ONLY_IF_RW { KEEP (*(.eh_frame)) }
   .gcc_except_table   : ONLY_IF_RW { *(.gcc_except_table .gcc_except_table.*) }