lguest: suppress interrupts for single insn, not range.

The last patch reduced our interrupt-suppression region to one address,
so simplify the code somewhat.

Also, remove the obsolete undefined instruction ranges and the comment
which refers to lguest_guest.S instead of head_32.S.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

arch/x86/include/asm/lguest.h

@@ -20,13 +20,10 @@ extern unsigned long switcher_addr;
 /* Found in switcher.S */
 extern unsigned long default_idt_entries[];
 
-/* Declarations for definitions in lguest_guest.S */
-extern char lguest_noirq_start[], lguest_noirq_end[];
+/* Declarations for definitions in arch/x86/lguest/head_32.S */
+extern char lguest_noirq_iret[];
 extern const char lgstart_cli[], lgend_cli[];
-extern const char lgstart_sti[], lgend_sti[];
-extern const char lgstart_popf[], lgend_popf[];
 extern const char lgstart_pushf[], lgend_pushf[];
-extern const char lgstart_iret[], lgend_iret[];
 
 extern void lguest_iret(void);
 extern void lguest_init(void);

arch/x86/lguest/boot.c

@@ -87,8 +87,7 @@
 
 struct lguest_data lguest_data = {
 	.hcall_status = { [0 ... LHCALL_RING_SIZE-1] = 0xFF },
-	.noirq_start = (u32)lguest_noirq_start,
-	.noirq_end = (u32)lguest_noirq_end,
+	.noirq_iret = (u32)lguest_noirq_iret,
 	.kernel_address = PAGE_OFFSET,
 	.blocked_interrupts = { 1 }, /* Block timer interrupts */
 	.syscall_vec = SYSCALL_VECTOR,

arch/x86/lguest/head_32.S

@@ -133,9 +133,8 @@ ENTRY(lg_restore_fl)
 	ret
 /*:*/
 
-/* These demark the EIP range where host should never deliver interrupts. */
-.global lguest_noirq_start
-.global lguest_noirq_end
+/* These demark the EIP where host should never deliver interrupts. */
+.global lguest_noirq_iret
 
 /*M:004
  * When the Host reflects a trap or injects an interrupt into the Guest, it
@@ -174,12 +173,11 @@ ENTRY(lg_restore_fl)
  *
  * The second is harder: copying eflags to lguest_data.irq_enabled will turn
  * interrupts on before we're finished, so we could be interrupted before we
- * return to userspace or wherever.  Our solution to this is to surround the
- * code with lguest_noirq_start: and lguest_noirq_end: labels.  We tell the
+ * return to userspace or wherever.  Our solution to this is to tell the
  * Host that it is *never* to interrupt us there, even if interrupts seem to be
  * enabled. (It's not necessary to protect pop instruction, since
- * data gets updated only after it completes, so we end up surrounding
- * just one instruction, iret).
+ * data gets updated only after it completes, so we only need to protect
+ * one instruction, iret).
  */
 ENTRY(lguest_iret)
 	pushl	2*4(%esp)
@@ -190,6 +188,5 @@ ENTRY(lguest_iret)
 	 * prefix makes sure we use the stack segment, which is still valid.
 	 */
 	popl	%ss:lguest_data+LGUEST_DATA_irq_enabled
-lguest_noirq_start:
+lguest_noirq_iret:
 	iret
-lguest_noirq_end:

drivers/lguest/hypercalls.c

@@ -211,10 +211,9 @@ static void initialize(struct lg_cpu *cpu)
 
 	/*
 	 * The Guest tells us where we're not to deliver interrupts by putting
-	 * the range of addresses into "struct lguest_data".
+	 * the instruction address into "struct lguest_data".
 	 */
-	if (get_user(cpu->lg->noirq_start, &cpu->lg->lguest_data->noirq_start)
-	    || get_user(cpu->lg->noirq_end, &cpu->lg->lguest_data->noirq_end))
+	if (get_user(cpu->lg->noirq_iret, &cpu->lg->lguest_data->noirq_iret))
 		kill_guest(cpu, "bad guest page %p", cpu->lg->lguest_data);
 
 	/*

drivers/lguest/interrupts_and_traps.c

@@ -204,8 +204,7 @@ void try_deliver_interrupt(struct lg_cpu *cpu, unsigned int irq, bool more)
 	 * They may be in the middle of an iret, where they asked us never to
 	 * deliver interrupts.
 	 */
-	if (cpu->regs->eip >= cpu->lg->noirq_start &&
-	   (cpu->regs->eip < cpu->lg->noirq_end))
+	if (cpu->regs->eip == cpu->lg->noirq_iret)
 		return;
 
 	/* If they're halted, interrupts restart them. */
@@ -395,8 +394,9 @@ static bool direct_trap(unsigned int num)
  * The Guest has the ability to turn its interrupt gates into trap gates,
  * if it is careful.  The Host will let trap gates can go directly to the
  * Guest, but the Guest needs the interrupts atomically disabled for an
- * interrupt gate.  It can do this by pointing the trap gate at instructions
- * within noirq_start and noirq_end, where it can safely disable interrupts.
+ * interrupt gate.  The Host could provide a mechanism to register more
+ * "no-interrupt" regions, and the Guest could point the trap gate at
+ * instructions within that region, where it can safely disable interrupts.
  */
 
 /*M:006

drivers/lguest/lg.h

@@ -102,7 +102,7 @@ struct lguest {
 
 	struct pgdir pgdirs[4];
 
-	unsigned long noirq_start, noirq_end;
+	unsigned long noirq_iret;
 
 	unsigned int stack_pages;
 	u32 tsc_khz;

include/linux/lguest.h

@@ -61,8 +61,8 @@ struct lguest_data {
 	u32 tsc_khz;
 
 /* Fields initialized by the Guest at boot: */
-	/* Instruction range to suppress interrupts even if enabled */
-	unsigned long noirq_start, noirq_end;
+	/* Instruction to suppress interrupts even if enabled */
+	unsigned long noirq_iret;
 	/* Address above which page tables are all identical. */
 	unsigned long kernel_address;
 	/* The vector to try to use for system calls (0x40 or 0x80). */