x86/xen: switch initial pvops IRQ functions to dummy ones

The initial pvops functions handling irq flags will only ever be called
before interrupts are being enabled.

So switch them to be dummy functions:
- xen_save_fl() can always return 0
- xen_irq_disable() is a nop
- xen_irq_enable() can BUG()

Add some generic paravirt functions for that purpose.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211028072748.29862-3-jgross@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/include/asm/paravirt_types.h

@@ -577,7 +577,9 @@ void paravirt_leave_lazy_mmu(void);
 void paravirt_flush_lazy_mmu(void);
 
 void _paravirt_nop(void);
+void paravirt_BUG(void);
 u64 _paravirt_ident_64(u64);
+unsigned long paravirt_ret0(void);
 
 #define paravirt_nop	((void *)_paravirt_nop)
 

arch/x86/kernel/paravirt.c

@@ -46,6 +46,17 @@ asm (".pushsection .entry.text, \"ax\"\n"
      ".type _paravirt_nop, @function\n\t"
      ".popsection");
 
+/* stub always returning 0. */
+asm (".pushsection .entry.text, \"ax\"\n"
+     ".global paravirt_ret0\n"
+     "paravirt_ret0:\n\t"
+     "xor %" _ASM_AX ", %" _ASM_AX ";\n\t"
+     "ret\n\t"
+     ".size paravirt_ret0, . - paravirt_ret0\n\t"
+     ".type paravirt_ret0, @function\n\t"
+     ".popsection");
+
+
 void __init default_banner(void)
 {
 	printk(KERN_INFO "Booting paravirtualized kernel on %s\n",
@@ -53,7 +64,7 @@ void __init default_banner(void)
 }
 
 /* Undefined instruction for dealing with missing ops pointers. */
-static void paravirt_BUG(void)
+noinstr void paravirt_BUG(void)
 {
 	BUG();
 }

arch/x86/xen/enlighten.c

@@ -31,25 +31,10 @@ EXPORT_SYMBOL_GPL(hypercall_page);
  * Pointer to the xen_vcpu_info structure or
  * &HYPERVISOR_shared_info->vcpu_info[cpu]. See xen_hvm_init_shared_info
  * and xen_vcpu_setup for details. By default it points to share_info->vcpu_info
- * but if the hypervisor supports VCPUOP_register_vcpu_info then it can point
- * to xen_vcpu_info. The pointer is used in __xen_evtchn_do_upcall to
- * acknowledge pending events.
- * Also more subtly it is used by the patched version of irq enable/disable
- * e.g. xen_irq_enable_direct and xen_iret in PV mode.
- *
- * The desire to be able to do those mask/unmask operations as a single
- * instruction by using the per-cpu offset held in %gs is the real reason
- * vcpu info is in a per-cpu pointer and the original reason for this
- * hypercall.
- *
+ * but during boot it is switched to point to xen_vcpu_info.
+ * The pointer is used in __xen_evtchn_do_upcall to acknowledge pending events.
  */
 DEFINE_PER_CPU(struct vcpu_info *, xen_vcpu);
-
-/*
- * Per CPU pages used if hypervisor supports VCPUOP_register_vcpu_info
- * hypercall. This can be used both in PV and PVHVM mode. The structure
- * overrides the default per_cpu(xen_vcpu, cpu) value.
- */
 DEFINE_PER_CPU(struct vcpu_info, xen_vcpu_info);
 
 /* Linux <-> Xen vCPU id mapping */

arch/x86/xen/irq.c

@@ -24,60 +24,6 @@ noinstr void xen_force_evtchn_callback(void)
 	(void)HYPERVISOR_xen_version(0, NULL);
 }
 
-asmlinkage __visible noinstr unsigned long xen_save_fl(void)
-{
-	struct vcpu_info *vcpu;
-	unsigned long flags;
-
-	vcpu = this_cpu_read(xen_vcpu);
-
-	/* flag has opposite sense of mask */
-	flags = !vcpu->evtchn_upcall_mask;
-
-	/* convert to IF type flag
-	   -0 -> 0x00000000
-	   -1 -> 0xffffffff
-	*/
-	return (-flags) & X86_EFLAGS_IF;
-}
-__PV_CALLEE_SAVE_REGS_THUNK(xen_save_fl, ".noinstr.text");
-
-asmlinkage __visible noinstr void xen_irq_disable(void)
-{
-	/* There's a one instruction preempt window here.  We need to
-	   make sure we're don't switch CPUs between getting the vcpu
-	   pointer and updating the mask. */
-	preempt_disable();
-	this_cpu_read(xen_vcpu)->evtchn_upcall_mask = 1;
-	preempt_enable_no_resched();
-}
-__PV_CALLEE_SAVE_REGS_THUNK(xen_irq_disable, ".noinstr.text");
-
-asmlinkage __visible noinstr void xen_irq_enable(void)
-{
-	struct vcpu_info *vcpu;
-
-	/*
-	 * We may be preempted as soon as vcpu->evtchn_upcall_mask is
-	 * cleared, so disable preemption to ensure we check for
-	 * events on the VCPU we are still running on.
-	 */
-	preempt_disable();
-
-	vcpu = this_cpu_read(xen_vcpu);
-	vcpu->evtchn_upcall_mask = 0;
-
-	/* Doesn't matter if we get preempted here, because any
-	   pending event will get dealt with anyway. */
-
-	barrier(); /* unmask then check (avoid races) */
-	if (unlikely(vcpu->evtchn_upcall_pending))
-		xen_force_evtchn_callback();
-
-	preempt_enable();
-}
-__PV_CALLEE_SAVE_REGS_THUNK(xen_irq_enable, ".noinstr.text");
-
 static void xen_safe_halt(void)
 {
 	/* Blocking includes an implicit local_irq_enable(). */
@@ -96,10 +42,10 @@ static void xen_halt(void)
 
 static const typeof(pv_ops) xen_irq_ops __initconst = {
 	.irq = {
-
-		.save_fl = PV_CALLEE_SAVE(xen_save_fl),
-		.irq_disable = PV_CALLEE_SAVE(xen_irq_disable),
-		.irq_enable = PV_CALLEE_SAVE(xen_irq_enable),
+		/* Initial interrupt flag handling only called while interrupts off. */
+		.save_fl = __PV_IS_CALLEE_SAVE(paravirt_ret0),
+		.irq_disable = __PV_IS_CALLEE_SAVE(paravirt_nop),
+		.irq_enable = __PV_IS_CALLEE_SAVE(paravirt_BUG),
 
 		.safe_halt = xen_safe_halt,
 		.halt = xen_halt,