libpayload/x86/exception: Add ability to handle user defined interrupts

I need to setup the APIC timer to fire interrupts. I would like to reuse
the existing interrupt table. So I extended it to support user defined
interrupts. I just added all 255 vectors so there wouldn't need to be
any additional build time configuration.

I'm going to deprecate exception_install_hook and remove it in a follow
up. It will be replaced with set_interrupt_handler. This way the
exception lookup does not have to manage a list of callbacks, or have to
worry about the order they are processed.

BUG=b:109749762
TEST=Wrote an interrupt handler and fired an APIC timer interrupt and
verified that vector 32 was returned.

Change-Id: Id9c2583c7c3d9be4a06a25e546e64399f2b0620c
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/28100
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/libpayload/arch/x86/exception.c

@@ -37,6 +37,9 @@
 u32 exception_stack[0x400] __attribute__((aligned(8)));
 
 static exception_hook hook;
+
+static interrupt_handler handlers[256];
+
 static const char *names[EXC_COUNT] = {
 	[EXC_DE]  = "Divide by Zero",
 	[EXC_DB]  = "Debug",
@@ -163,7 +166,16 @@ static void dump_exception_state(void)
 void exception_dispatch(void)
 {
 	u32 vec = exception_state->vector;
-	die_if(vec >= EXC_COUNT || !names[vec], "Bad exception vector %u", vec);
+
+	die_if(vec >= ARRAY_SIZE(handlers), "Invalid vector %u\n", vec);
+
+	if (handlers[vec]) {
+		handlers[vec](vec);
+		return;
+	}
+
+	die_if(vec >= EXC_COUNT || !names[vec], "Bad exception vector %u\n",
+	       vec);
 
 	if (hook && hook(vec))
 		return;
@@ -185,6 +197,11 @@ void exception_install_hook(exception_hook h)
 	hook = h;
 }
 
+void set_interrupt_handler(u8 vector, interrupt_handler handler)
+{
+	handlers[vector] = handler;
+}
+
 static uint32_t eflags(void)
 {
 	uint32_t eflags;