[trace] Update the registers for the trace reporting calling convention

- Use registers r12, r13 and r14 to pass trace data information from the
  guest to the host
- This is in use temporarily, until the virtio queues are implemented

Signed-off-by: Doru Blânzeanu <dblnz@pm.me>

Author: dblnz

src/hyperlight_guest/src/exit.rs

@@ -103,9 +103,9 @@ pub(crate) unsafe fn out32(port: u16, val: u32) {
                 asm!("out dx, eax",
                     in("dx") port,
                     in("eax") val,
-                    in("r8") OutBAction::TraceBatch as u64,
-                    in("r9") ptr,
-                    in("r10") len,
+                    in("r12") OutBAction::TraceBatch as u64,
+                    in("r13") ptr,
+                    in("r14") len,
                     options(preserves_flags, nomem, nostack)
                 )
             };

src/hyperlight_guest_tracing/src/state.rs

@@ -54,9 +54,9 @@ fn send_to_host(data: &[u8]) {
             in("dx") OutBAction::TraceBatch as u16,
             in("al") 0u8,
             // Additional magic number to identify the action
-            in("r8") OutBAction::TraceBatch as u64,
-            in("r9") data.as_ptr() as u64,
-            in("r10") data.len() as u64,
+            in("r12") OutBAction::TraceBatch as u64,
+            in("r13") data.as_ptr() as u64,
+            in("r14") data.len() as u64,
         );
     }
 }

src/hyperlight_host/src/sandbox/trace/context.rs

@@ -41,22 +41,22 @@ impl EventsBatch {
     /// Extract a batch of guest trace events from guest memory.
     ///
     /// The guest passes the trace data pointer as a Guest Virtual Address (GVA)
-    /// in register r9. With Copy-on-Write enabled, this GVA may not be
+    /// in register r13. With Copy-on-Write enabled, this GVA may not be
     /// identity-mapped to its physical address, so we walk the guest page
     /// tables to translate GVA → GPA before reading the data.
     ///
     /// # Arguments
-    /// * `regs` - The guest registers (r8 = magic, r9 = GVA pointer, r10 = length)
+    /// * `regs` - The guest registers (r12 = magic, r13 = GVA pointer, r14 = length)
     /// * `mem_mgr` - The sandbox memory manager with access to shared and scratch memory
     /// * `root_pt` - The root page table physical address (CR3) for GVA translation
     fn from_regs(
         regs: &CommonRegisters,
         mem_mgr: &mut SandboxMemoryManager<HostSharedMemory>,
         root_pt: u64,
     ) -> Result<Self> {
-        let magic_no = regs.r8;
-        let trace_data_ptr = regs.r9 as usize;
-        let trace_data_len = regs.r10 as usize;
+        let magic_no = regs.r12;
+        let trace_data_gva = regs.r13;
+        let trace_data_len = regs.r14 as usize;
 
         // Validate the magic number to ensure the guest is providing trace data
         if magic_no != OutBAction::TraceBatch as u64 {