x86/xen: Support early interrupts in xen pv guests

Add early interrupt handlers activated by idt_setup_early_handler() to
the handlers supported by Xen pv guests. This will allow for early
WARN() calls not crashing the guest.
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel@lists.xenproject.org
Cc: boris.ostrovsky@oracle.com
Link: https://lkml.kernel.org/r/20171124084221.30172-1-jgross@suse.com

arch/x86/include/asm/segment.h

@@ -236,11 +236,23 @@
  */
 #define EARLY_IDT_HANDLER_SIZE 9
 
+/*
+ * xen_early_idt_handler_array is for Xen pv guests: for each entry in
+ * early_idt_handler_array it contains a prequel in the form of
+ * pop %rcx; pop %r11; jmp early_idt_handler_array[i]; summing up to
+ * max 8 bytes.
+ */
+#define XEN_EARLY_IDT_HANDLER_SIZE 8
+
 #ifndef __ASSEMBLY__
 
 extern const char early_idt_handler_array[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_SIZE];
 extern void early_ignore_irq(void);
 
+#if defined(CONFIG_X86_64) && defined(CONFIG_XEN_PV)
+extern const char xen_early_idt_handler_array[NUM_EXCEPTION_VECTORS][XEN_EARLY_IDT_HANDLER_SIZE];
+#endif
+
 /*
  * Load a segment. Fall back on loading the zero segment if something goes
  * wrong.  This variant assumes that loading zero fully clears the segment.

arch/x86/mm/extable.c

 #include <linux/extable.h>
 #include <linux/uaccess.h>
 #include <linux/sched/debug.h>
+#include <xen/xen.h>
 
 #include <asm/fpu/internal.h>
 #include <asm/traps.h>
@@ -212,8 +213,9 @@ void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
 	 * Old CPUs leave the high bits of CS on the stack
 	 * undefined.  I'm not sure which CPUs do this, but at least
 	 * the 486 DX works this way.
+	 * Xen pv domains are not using the default __KERNEL_CS.
 	 */
-	if (regs->cs != __KERNEL_CS)
+	if (!xen_pv_domain() && regs->cs != __KERNEL_CS)
 		goto fail;
 
 	/*

arch/x86/xen/enlighten_pv.c

@@ -622,7 +622,7 @@ static struct trap_array_entry trap_array[] = {
 	{ simd_coprocessor_error,      xen_simd_coprocessor_error,      false },
 };
 
-static bool get_trap_addr(void **addr, unsigned int ist)
+static bool __ref get_trap_addr(void **addr, unsigned int ist)
 {
 	unsigned int nr;
 	bool ist_okay = false;
@@ -644,6 +644,14 @@ static bool get_trap_addr(void **addr, unsigned int ist)
 		}
 	}
 
+	if (nr == ARRAY_SIZE(trap_array) &&
+	    *addr >= (void *)early_idt_handler_array[0] &&
+	    *addr < (void *)early_idt_handler_array[NUM_EXCEPTION_VECTORS]) {
+		nr = (*addr - (void *)early_idt_handler_array[0]) /
+		     EARLY_IDT_HANDLER_SIZE;
+		*addr = (void *)xen_early_idt_handler_array[nr];
+	}
+
 	if (WARN_ON(ist != 0 && !ist_okay))
 		return false;
 
@@ -1262,6 +1270,21 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	xen_setup_gdt(0);
 
 	xen_init_irq_ops();
+
+	/* Let's presume PV guests always boot on vCPU with id 0. */
+	per_cpu(xen_vcpu_id, 0) = 0;
+
+	/*
+	 * Setup xen_vcpu early because idt_setup_early_handler needs it for
+	 * local_irq_disable(), irqs_disabled().
+	 *
+	 * Don't do the full vcpu_info placement stuff until we have
+	 * the cpu_possible_mask and a non-dummy shared_info.
+	 */
+	xen_vcpu_info_reset(0);
+
+	idt_setup_early_handler();
+
 	xen_init_capabilities();
 
 #ifdef CONFIG_X86_LOCAL_APIC
@@ -1295,18 +1318,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
 	 */
 	acpi_numa = -1;
 #endif
-	/* Let's presume PV guests always boot on vCPU with id 0. */
-	per_cpu(xen_vcpu_id, 0) = 0;
-
-	/*
-	 * Setup xen_vcpu early because start_kernel needs it for
-	 * local_irq_disable(), irqs_disabled().
-	 *
-	 * Don't do the full vcpu_info placement stuff until we have
-	 * the cpu_possible_mask and a non-dummy shared_info.
-	 */
-	xen_vcpu_info_reset(0);
-
 	WARN_ON(xen_cpuhp_setup(xen_cpu_up_prepare_pv, xen_cpu_dead_pv));
 
 	local_irq_disable();

arch/x86/xen/xen-asm_64.S

@@ -15,6 +15,7 @@
 
 #include <xen/interface/xen.h>
 
+#include <linux/init.h>
 #include <linux/linkage.h>
 
 .macro xen_pv_trap name
@@ -54,6 +55,19 @@ xen_pv_trap entry_INT80_compat
 #endif
 xen_pv_trap hypervisor_callback
 
+	__INIT
+ENTRY(xen_early_idt_handler_array)
+	i = 0
+	.rept NUM_EXCEPTION_VECTORS
+	pop %rcx
+	pop %r11
+	jmp early_idt_handler_array + i*EARLY_IDT_HANDLER_SIZE
+	i = i + 1
+	.fill xen_early_idt_handler_array + i*XEN_EARLY_IDT_HANDLER_SIZE - ., 1, 0xcc
+	.endr
+END(xen_early_idt_handler_array)
+	__FINIT
+
 hypercall_iret = hypercall_page + __HYPERVISOR_iret * 32
 /*
  * Xen64 iret frame: