Index: sys/amd64/amd64/mp_machdep.c
===================================================================
--- sys/amd64/amd64/mp_machdep.c
+++ sys/amd64/amd64/mp_machdep.c
@@ -103,6 +103,7 @@
 static char *mce_stack;
 static char *nmi_stack;
 static char *dbg_stack;
+void *bootpcpu;
 
 extern u_int mptramp_la57;
 extern u_int mptramp_nx;
@@ -197,10 +198,8 @@
 	/* Update microcode before doing anything else. */
 	ucode_load_ap(cpu);
 
-	/* Get per-cpu data and save  */
-	pc = &__pcpu[cpu];
-
-	/* prime data page for it to use */
+	/* Initialize the PCPU area. */
+	pc = bootpcpu;
 	pcpu_init(pc, cpu, sizeof(struct pcpu));
 	dpcpu_init(dpcpu, cpu);
 	pc->pc_apic_id = cpu_apic_ids[cpu];
@@ -262,8 +261,8 @@
 	lgdt(&ap_gdt);			/* does magic intra-segment return */
 
 	wrmsr(MSR_FSBASE, 0);		/* User value */
-	wrmsr(MSR_GSBASE, (u_int64_t)pc);
-	wrmsr(MSR_KGSBASE, (u_int64_t)pc);	/* XXX User value while we're in the kernel */
+	wrmsr(MSR_GSBASE, (uint64_t)pc);
+	wrmsr(MSR_KGSBASE, 0);		/* User value */
 	fix_cpuid();
 
 	lidt(&r_idt);
@@ -431,6 +430,7 @@
 		dpcpu = (void *)kmem_malloc_domainset(DOMAINSET_PREF(domain),
 		    DPCPU_SIZE, M_WAITOK | M_ZERO);
 
+		bootpcpu = &__pcpu[cpu];
 		bootSTK = (char *)bootstacks[cpu] +
 		    kstack_pages * PAGE_SIZE - 8;
 		bootAP = cpu;
Index: sys/amd64/amd64/mpboot.S
===================================================================
--- sys/amd64/amd64/mpboot.S
+++ sys/amd64/amd64/mpboot.S
@@ -260,4 +260,21 @@
 	.p2align 4,0
 entry_64:
 	movq	bootSTK, %rsp
+
+	/*
+	 * Initialize the segment register used for the PCPU area.  The PCPU
+	 * area will be initialized by init_secondary(), but it should be
+	 * accessible before that to support sanitizer instrumentation which
+	 * accesses per-CPU variables.
+	 *
+	 * Note that GS.base is loaded again in init_secondary().  This is not
+	 * redundant: lgdt() loads a selector into %gs and this has the side
+	 * effect of clearing GS.base.
+	 */
+	movl	$MSR_GSBASE, %ecx
+	movq	bootpcpu, %rax
+	movq	%rax, %rdx
+	shrq	$32, %rdx
+	wrmsr
+
 	jmp	init_secondary