Any application that wants to receive a user interrupt needs to register
an interrupt handler with the kernel. Add a registration syscall that
sets up the interrupt handler and the related kernel structures for
the task that makes this syscall.

Only one interrupt handler per task can be registered with the
kernel/hardware. Each task has its private interrupt vector space of 64
vectors. The vector registration and the related FD management is
covered later.

Also add an unregister syscall to let a task unregister the interrupt
handler.

The UPID for each receiver task needs to be updated whenever a task gets
context switched or it moves from one cpu to another. This will also be
covered later. The system calls haven't been wired up yet so no real
harm is done if we don't update the UPID right now.

<Code typically in the x86/kernel directory doesn't deal with file
descriptor management. I have kept uintr_fd.c separate to make it easier
to move it somewhere else if needed.>

Signed-off-by: Jacob Pan <redacted>
Signed-off-by: Sohil Mehta <redacted>
---
 arch/x86/include/asm/processor.h |   6 +
 arch/x86/include/asm/uintr.h     |  13 ++
 arch/x86/kernel/Makefile         |   1 +
 arch/x86/kernel/uintr_core.c     | 240 +++++++++++++++++++++++++++++++
 arch/x86/kernel/uintr_fd.c       |  58 ++++++++
 5 files changed, 318 insertions(+)
 create mode 100644 arch/x86/include/asm/uintr.h
 create mode 100644 arch/x86/kernel/uintr_core.c
 create mode 100644 arch/x86/kernel/uintr_fd.c

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 9ad2acaaae9b..d229bfac8b4f 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h

@@ -9,6 +9,7 @@ struct task_struct;
 struct mm_struct;
 struct io_bitmap;
 struct vm86;
+struct uintr_receiver;
 
 #include <asm/math_emu.h>
 #include <asm/segment.h>

@@ -529,6 +530,11 @@ struct thread_struct {
 	 */
 	u32			pkru;
 
+#ifdef CONFIG_X86_USER_INTERRUPTS
+	/* User Interrupt state*/
+	struct uintr_receiver	*ui_recv;
+#endif
+
 	/* Floating point and extended processor state */
 	struct fpu		fpu;
 	/*

diff --git a/arch/x86/include/asm/uintr.h b/arch/x86/include/asm/uintr.h
new file mode 100644
index 000000000000..4f35bd8bd4e0
--- /dev/null
+++ b/arch/x86/include/asm/uintr.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_UINTR_H
+#define _ASM_X86_UINTR_H
+
+#ifdef CONFIG_X86_USER_INTERRUPTS
+
+bool uintr_arch_enabled(void);
+int do_uintr_register_handler(u64 handler);
+int do_uintr_unregister_handler(void);
+
+#endif /* CONFIG_X86_USER_INTERRUPTS */
+
+#endif /* _ASM_X86_UINTR_H */

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 8f4e8fa6ed75..060ca9f23e23 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile

@@ -140,6 +140,7 @@ obj-$(CONFIG_UPROBES)			+= uprobes.o
 obj-$(CONFIG_PERF_EVENTS)		+= perf_regs.o
 obj-$(CONFIG_TRACING)			+= tracepoint.o
 obj-$(CONFIG_SCHED_MC_PRIO)		+= itmt.o
+obj-$(CONFIG_X86_USER_INTERRUPTS)	+= uintr_fd.o uintr_core.o
 obj-$(CONFIG_X86_UMIP)			+= umip.o
 
 obj-$(CONFIG_UNWINDER_ORC)		+= unwind_orc.o

diff --git a/arch/x86/kernel/uintr_core.c b/arch/x86/kernel/uintr_core.c
new file mode 100644
index 000000000000..2c6042a6840a
--- /dev/null
+++ b/arch/x86/kernel/uintr_core.c

@@ -0,0 +1,240 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2021, Intel Corporation.
+ *
+ * Sohil Mehta <sohil.mehta@intel.com>
+ * Jacob Pan <jacob.jun.pan@linux.intel.com>
+ */
+#define pr_fmt(fmt)    "uintr: " fmt
+
+#include <linux/refcount.h>
+#include <linux/sched.h>
+#include <linux/sched/task.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+
+#include <asm/apic.h>
+#include <asm/fpu/internal.h>
+#include <asm/irq_vectors.h>
+#include <asm/msr.h>
+#include <asm/msr-index.h>
+#include <asm/uintr.h>
+
+/* User Posted Interrupt Descriptor (UPID) */
+struct uintr_upid {
+	struct {
+		u8 status;	/* bit 0: ON, bit 1: SN, bit 2-7: reserved */
+		u8 reserved1;	/* Reserved */
+		u8 nv;		/* Notification vector */
+		u8 reserved2;	/* Reserved */