some fixes for the #HV handler

kernel/src/cpu/idt/entry.S

@@ -117,7 +117,7 @@ hv_not_vmpl_switch:
 	// point, there are two possibilities.  If RIP is before the IRET
 	// instruction itself, then the RSP at the time of #HV exception
 	// points to the register context that was established for the previous
-	// exceptoin.  In that case, the current RSP can be changed to point
+	// exception.  In that case, the current RSP can be changed to point
 	// to that exception context, and the #HV can be handled using that
 	// register context, and when #HV processing completes, the subsequent
 	// end-of-interrupt flow will restore the context at the time of the
@@ -171,7 +171,7 @@ restart_hv:
 	// register state at the time of the exception that was returning at
 	// the time the #HV arrived.
 	// At this point, RCX holds the stack pointer at the time of the
-	// IRET taht was aborted.  The first QWORD below that pointer is
+	// IRET that was aborted.  The first QWORD below that pointer is
 	// reserved for the dummy error code, then the three QWORDS below that
 	// will hold the RAX, RBX, and RCX values, which are presently stored
 	// in the top three QWORDs of the current stack.
@@ -185,7 +185,7 @@ restart_hv:
 
 continue_hv:
 	// At this point, only the dummy error code and first three registers
-	// have been pushed onto the stack.  Push the remainder o construct a
+	// have been pushed onto the stack.  Push the remainder to construct a
 	// full exception context.
         pushq   %rdx
         pushq   %rsi
@@ -213,19 +213,14 @@ handle_as_hv:
 	call 	process_hv_events
 	// fall through to default_return
 
-.globl return_new_task
-return_new_task:
-	call setup_new_task
-	jmp default_return
-
 .globl default_return
 default_return:
 	// Ensure that interrupts are disabled before attempting any return.
 	cli
 	testb	$3, 17*8(%rsp) // Check CS in exception frame
 	jnz 	return_user
 return_all_paths:
-	// If interrupts were prerviously available, then check whether any #HV
+	// If interrupts were previously available, then check whether any #HV
 	// events are pending.  If so, proceed as if the original trap was
 	// #HV.
 	testl 	$0x200, 18*8(%rsp) // check EFLAGS.IF in exception frame
@@ -273,6 +268,11 @@ return_user:
 	// Put user-mode specific return code here
 	jmp 	return_all_paths
 
+.globl return_new_task
+return_new_task:
+	call setup_new_task
+	jmp default_return
+
 // #DE Divide-by-Zero-Error Exception (Vector 0)
 default_entry_no_ist	name=de		handler=panic			error_code=0	vector=0
 

kernel/src/cpu/percpu.rs

@@ -309,7 +309,7 @@ pub struct PerCpu {
     ghcb: OnceCell<GhcbPage>,
 
     /// `#HV` doorbell page for this CPU.
-    hv_doorbell: OnceCell<&'static HVDoorbell>,
+    hv_doorbell: Cell<Option<&'static HVDoorbell>>,
 
     init_stack: Cell<Option<VirtAddr>>,
     ist: IstStacks,
@@ -341,7 +341,7 @@ impl PerCpu {
 
             shared: PerCpuShared::new(apic_id),
             ghcb: OnceCell::new(),
-            hv_doorbell: OnceCell::new(),
+            hv_doorbell: Cell::new(None),
             init_stack: Cell::new(None),
             ist: IstStacks::new(),
             current_stack: Cell::new(MemoryRegion::new(VirtAddr::null(), 0)),
@@ -408,17 +408,14 @@ impl PerCpu {
     }
 
     pub fn hv_doorbell(&self) -> Option<&'static HVDoorbell> {
-        self.hv_doorbell.get().copied()
+        self.hv_doorbell.get()
     }
 
     /// Gets a pointer to the location of the HV doorbell pointer in the
     /// PerCpu structure. Pointers and references have the same layout, so
     /// the return type is equivalent to `*const *const HVDoorbell`.
     pub fn hv_doorbell_addr(&self) -> *const &'static HVDoorbell {
-        self.hv_doorbell
-            .get()
-            .map(ptr::from_ref)
-            .unwrap_or(ptr::null())
+        self.hv_doorbell.as_ptr().cast()
     }
 
     pub fn get_top_of_stack(&self) -> VirtAddr {
@@ -488,9 +485,11 @@ impl PerCpu {
 
     fn setup_hv_doorbell(&self) -> Result<(), SvsmError> {
         let doorbell = allocate_hv_doorbell_page(current_ghcb())?;
-        self.hv_doorbell
-            .set(doorbell)
-            .expect("Attempted to reinitialize the HV doorbell page");
+        assert!(
+            self.hv_doorbell.get().is_none(),
+            "Attempted to reinitialize the HV doorbell page"
+        );
+        self.hv_doorbell.set(Some(doorbell));
         Ok(())
     }