feat(x86_64): display page fault address in oopses (#420)

Debugging page faults in Mycelium is currently a bit difficult, because
the oops message doesn't display the virtual address at which the page
fault occurred. We can fix this quite easily by reading the `cr2`
control register, which contains the linear address of the page fault.

hal-x86_64/src/control_regs.rs

@@ -1,4 +1,5 @@
 use core::arch::asm;
+use hal_core::VAddr;
 use mycelium_util::bits::bitfield;
 
 pub mod cr3 {
@@ -15,7 +16,7 @@ pub mod cr3 {
             asm!("mov {0}, cr3", out(reg) val, options(readonly));
         };
         let addr = PAddr::from_u64(val);
-        tracing::debug!(?addr);
+        tracing::trace!(rax = ?addr, "mov rax, cr3");
         let pml4_page = Page::starting_at_fixed(addr)
             .expect("PML4 physical addr not aligned! this is very bad");
         (pml4_page, Flags(val))
@@ -236,6 +237,20 @@ impl Cr4 {
     }
 }
 
+/// Control Register 2 (CR2) contains the Page Fault Linear Address (PFLA).
+pub struct Cr2;
+
+impl Cr2 {
+    /// Returns the 32-bit Page Fault Linear Address (PFLA) stored in CR2.
+    pub fn read() -> VAddr {
+        let addr: u64;
+        unsafe {
+            asm!("mov {0}, cr2", out(reg) addr, options(readonly));
+        };
+        VAddr::from_u64(addr)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

hal-x86_64/src/interrupt.rs

@@ -269,7 +269,7 @@ impl<'a, T> hal_core::interrupt::Context for Context<'a, T> {
 
 impl<'a> ctx::PageFault for Context<'a, PageFaultCode> {
     fn fault_vaddr(&self) -> crate::VAddr {
-        unimplemented!("eliza")
+        crate::control_regs::Cr2::read()
     }
 
     fn debug_error_code(&self) -> &dyn fmt::Debug {

src/arch/x86_64/interrupt.rs

@@ -71,10 +71,12 @@ impl hal_core::interrupt::Handlers<Registers> for InterruptHandlers {
     where
         C: interrupt::Context<Registers = Registers> + hal_core::interrupt::ctx::PageFault,
     {
+        let fault_vaddr = cx.fault_vaddr();
+        let code = cx.display_error_code();
         oops(Oops::fault_with_details(
             &cx,
             "PAGE FAULT",
-            &format_args!("\n{}", cx.display_error_code()),
+            &format_args!("at {fault_vaddr:?}\n{code}"),
         ))
     }