cpu/percpu: remove spinlock around pagetable

The PerCpu structure can only be accessed from the current CPU, so
there is no need for locking. Moreover, if the pagetable is acquired
more than once this will cause a deadlock. Replace the spinlock
instead with a RefCell, and return a RefMut from
`PerCpu::get_pgtable()`. This way, if we attempt to acquire more than
one coexisting reference to the per-cpu pagetable more than once, we
will panic immediately.

Signed-off-by: Carlos López <[email protected]>

kernel/src/cpu/percpu.rs

@@ -34,7 +34,7 @@ use crate::types::{PAGE_SHIFT, PAGE_SHIFT_2M, PAGE_SIZE, PAGE_SIZE_2M, SVSM_TR_F
 use crate::utils::MemoryRegion;
 use alloc::sync::Arc;
 use alloc::vec::Vec;
-use core::cell::{Cell, RefCell, UnsafeCell};
+use core::cell::{Cell, RefCell, RefMut, UnsafeCell};
 use core::mem::size_of;
 use core::ptr;
 use core::sync::atomic::{AtomicBool, Ordering};
@@ -256,7 +256,7 @@ pub struct PerCpu {
     shared: PerCpuShared,
 
     apic_id: u32,
-    pgtbl: SpinLock<PageTableRef>,
+    pgtbl: RefCell<PageTableRef>,
     tss: Cell<X86Tss>,
     svsm_vmsa: Cell<Option<VmsaRef>>,
     reset_ip: Cell<u64>,
@@ -284,7 +284,7 @@ impl PerCpu {
     fn new(apic_id: u32) -> Self {
         Self {
             apic_id,
-            pgtbl: SpinLock::new(PageTableRef::unset()),
+            pgtbl: RefCell::new(PageTableRef::unset()),
             tss: Cell::new(X86Tss::new()),
             svsm_vmsa: Cell::new(None),
             reset_ip: Cell::new(0xffff_fff0),
@@ -366,8 +366,7 @@ impl PerCpu {
     }
 
     pub fn set_pgtable(&self, pgtable: PageTableRef) {
-        let mut my_pgtable = self.get_pgtable();
-        *my_pgtable = pgtable;
+        *self.get_pgtable() = pgtable;
     }
 
     fn allocate_stack(&self, base: VirtAddr) -> Result<VirtAddr, SvsmError> {
@@ -392,8 +391,8 @@ impl PerCpu {
         Ok(())
     }
 
-    pub fn get_pgtable(&self) -> LockGuard<'_, PageTableRef> {
-        self.pgtbl.lock()
+    pub fn get_pgtable(&self) -> RefMut<'_, PageTableRef> {
+        self.pgtbl.borrow_mut()
     }
 
     /// Registers an already set up GHCB page for this CPU.