x86/acrn: Introduce acrn_{setup, remove}_intr_handler()

The ACRN Hypervisor builds an I/O request when a trapped I/O access
happens in User VM. Then, ACRN Hypervisor issues an upcall by sending
a notification interrupt to the Service VM. HSM in the Service VM needs
to hook the notification interrupt to handle I/O requests.

Notification interrupts from ACRN Hypervisor are already supported and
a, currently uninitialized, callback called.

Export two APIs for HSM to setup/remove its callback.

Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Fengwei Yin <fengwei.yin@intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Yu Wang <yu1.wang@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Originally-by: Yakui Zhao <yakui.zhao@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Shuo Liu <shuo.a.liu@intel.com>
Link: https://lore.kernel.org/r/20210207031040.49576-3-shuo.a.liu@intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/include/asm/acrn.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_ACRN_H
+#define _ASM_X86_ACRN_H
+
+void acrn_setup_intr_handler(void (*handler)(void));
+void acrn_remove_intr_handler(void);
+
+#endif /* _ASM_X86_ACRN_H */

arch/x86/kernel/cpu/acrn.c

@@ -10,6 +10,8 @@
  */
 
 #include <linux/interrupt.h>
+
+#include <asm/acrn.h>
 #include <asm/apic.h>
 #include <asm/cpufeatures.h>
 #include <asm/desc.h>
@@ -55,6 +57,18 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_acrn_hv_callback)
 	set_irq_regs(old_regs);
 }
 
+void acrn_setup_intr_handler(void (*handler)(void))
+{
+	acrn_intr_handler = handler;
+}
+EXPORT_SYMBOL_GPL(acrn_setup_intr_handler);
+
+void acrn_remove_intr_handler(void)
+{
+	acrn_intr_handler = NULL;
+}
+EXPORT_SYMBOL_GPL(acrn_remove_intr_handler);
+
 const __initconst struct hypervisor_x86 x86_hyper_acrn = {
 	.name                   = "ACRN",
 	.detect                 = acrn_detect,