[hyperlight_host] exceptions: allow guests to register handlers

This commit adds a simple mechanism for guests to register exception
handlers. It does not support any kind of chaining, so there can only be
one exception handler per exception per guest, which should probably be
improved in the future. This will be used in hyperlight-wasm shortly.

Signed-off-by: Lucy Menon <168595099+syntactically@users.noreply.github.com>

Author: syntactically

src/hyperlight_guest_bin/src/exceptions/handler.rs

@@ -21,20 +21,89 @@ use hyperlight_common::flatbuffer_wrappers::guest_error::ErrorCode;
 use hyperlight_common::outb::Exception;
 use hyperlight_guest::exit::abort_with_code_and_message;
 
+use crate::paging;
+
+/// See AMD64 Architecture Programmer's Manual, Volume 2
+///     §8.9.3 Interrupt Stack Frame, pp. 283--284
+///       Figure 8-14: Long-Mode Stack After Interrupt---Same Privilege,
+///       Figure 8-15: Long-Mode Stack After Interrupt---Higher Privilege
+/// Subject to the proviso that we push a dummy error code of 0 for exceptions
+/// for which the processor does not provide one
+#[repr(C)]
+pub struct ExceptionInfo {
+    pub error_code: u64,
+    pub rip: u64,
+    pub cs: u64,
+    pub rflags: u64,
+    pub rsp: u64,
+    pub ss: u64,
+}
+const _: () = assert!(core::mem::offset_of!(ExceptionInfo, rip) == 8);
+const _: () = assert!(core::mem::offset_of!(ExceptionInfo, rsp) == 32);
+
+#[repr(C)]
+/// Saved context, pushed onto the stack by exception entry code
+pub struct Context {
+    /// in order: gs, fs, es
+    pub segments: [u64; 3],
+    pub fxsave: [u8; 512],
+    pub ds: u64,
+    /// no `rsp`, since the processor saved it
+    /// `rax` is at the top, `r15` the bottom
+    pub gprs: [u64; 15],
+}
+const _: () = assert!(size_of::<Context>() == 152 + 512);
+
+// TODO: This will eventually need to end up in a per-thread context,
+// when there are threads.
+pub static handlers: [core::sync::atomic::AtomicU64; 31] =
+    [const { core::sync::atomic::AtomicU64::new(0) }; 31];
+type handler_t = fn(n: u64, info: *mut ExceptionInfo, ctx: *mut Context, pf_addr: u64) -> bool;
+
 /// Exception handler
 #[unsafe(no_mangle)]
 pub extern "C" fn hl_exception_handler(
     stack_pointer: u64,
     exception_number: u64,
     page_fault_address: u64,
 ) {
+    let ctx = stack_pointer as *mut Context;
+    let exn_info = (stack_pointer + size_of::<Context>() as u64) as *mut ExceptionInfo;
+
     let exception = Exception::try_from(exception_number as u8).expect("Invalid exception number");
+
+    let saved_rip = unsafe { (&raw const (*exn_info).rip).read_volatile() };
+    let error_code = unsafe { (&raw const (*exn_info).error_code).read_volatile() };
+
     let msg = format!(
-        "Page Fault Address: {:#x}\n\
-            Stack Pointer: {:#x}",
-        page_fault_address, stack_pointer
+        "Exception vector: {:#}\n\
+         Faulting Instruction: {:#x}\n\
+         Page Fault Address: {:#x}\n\
+         Error code: {:#x}\n\
+         Stack Pointer: {:#x}",
+        exception_number, saved_rip, page_fault_address, error_code, stack_pointer
     );
 
+    // We don't presently have any need for user-defined interrupts,
+    // so we only support handlers for the architecture-defined
+    // vectors (0-31)
+    if exception_number < 31 {
+        let handler =
+            handlers[exception_number as usize].load(core::sync::atomic::Ordering::Acquire);
+        if handler != 0
+            && unsafe {
+                core::mem::transmute::<_, handler_t>(handler)(
+                    exception_number,
+                    exn_info,
+                    ctx,
+                    page_fault_address,
+                )
+            }
+        {
+            return;
+        }
+    }
+
     unsafe {
         abort_with_code_and_message(
             &[ErrorCode::GuestError as u8, exception as u8],

src/hyperlight_guest_bin/src/lib.rs

@@ -35,9 +35,9 @@ use spin::Once;
 
 // === Modules ===
 #[cfg(target_arch = "x86_64")]
-mod exceptions {
+pub mod exceptions {
     pub(super) mod gdt;
-    mod handler;
+    pub mod handler;
     mod idt;
     pub(super) mod idtr;
     mod interrupt_entry;