Implement the memory version system that will be necessary for LR/SC

src/mem.rs

@@ -6,7 +6,10 @@
 
 use std::sync::{
     Arc,
-    atomic::{AtomicU8, AtomicU16, AtomicU32, AtomicU64, Ordering::Relaxed},
+    atomic::{
+        AtomicU8, AtomicU16, AtomicU32, AtomicU64,
+        Ordering::{self, Relaxed},
+    },
 };
 
 use memmap2::MmapMut;
@@ -179,6 +182,7 @@ pub enum MemoryMappingType {
 
 pub struct Ram {
     buf: MmapMut,
+    version_counters: Arc<[AtomicU32]>,
 }
 
 #[cfg(target_endian = "big")]
@@ -191,9 +195,16 @@ impl Ram {
         }
         Ok(Self {
             buf: MmapMut::map_anon(size)?,
+            // SAFETY: We do not care about the initial version counts. Wrapping is fine. Only
+            // equality is ever checked for, not magnitude.
+            version_counters: unsafe {
+                Arc::new_uninit_slice(size.div_ceil(Self::VERSION_CHUNK_SIZE)).assume_init()
+            },
         })
     }
 
+    const VERSION_CHUNK_SIZE: usize = 64;
+
     pub fn buf_mut(&mut self) -> &mut [u8] {
         self.buf.as_mut()
     }
@@ -311,6 +322,9 @@ impl Ram {
             return Ok(());
         }
 
+        self.claim_addr_even(addr)
+            .ok_or_else(|| MemoryExceptionType::AccessFault.with_addr(addr))?;
+
         let index = (addr / 8) as usize;
         unsafe {
             self.buf_transmuted::<AtomicU64>()
@@ -336,6 +350,9 @@ impl Ram {
             return Ok(());
         }
 
+        self.claim_addr_even(addr)
+            .ok_or_else(|| MemoryExceptionType::AccessFault.with_addr(addr))?;
+
         let index = (addr / 4) as usize;
         unsafe {
             self.buf_transmuted::<AtomicU32>()
@@ -361,6 +378,9 @@ impl Ram {
             return Ok(());
         }
 
+        self.claim_addr_even(addr)
+            .ok_or_else(|| MemoryExceptionType::AccessFault.with_addr(addr))?;
+
         let index = (addr / 2) as usize;
         unsafe {
             self.buf_transmuted::<AtomicU16>()
@@ -375,6 +395,8 @@ impl Ram {
     }
     #[inline]
     pub fn write_byte(&self, addr: u64, value: u8) -> Result<(), MemoryException> {
+        self.claim_addr_even(addr)
+            .ok_or_else(|| MemoryExceptionType::AccessFault.with_addr(addr))?;
         self.buf_atomic()
             .get(addr as usize)
             .ok_or(MemoryException {
@@ -384,6 +406,58 @@ impl Ram {
             .store(value, Relaxed);
         Ok(())
     }
+
+    pub fn claim_addr_even<'a>(&'a self, addr: u64) -> Option<RamVersionClaim<'a>> {
+        let chunk_id = addr as usize / Self::VERSION_CHUNK_SIZE;
+        let chunk_counter = self.version_counters.get(chunk_id)?;
+        Some(RamVersionClaim::claim_even(&chunk_counter))
+    }
+}
+
+pub struct RamVersionClaim<'a> {
+    version_counter: &'a AtomicU32,
+    intial_version: u32,
+}
+
+impl<'a> RamVersionClaim<'a> {
+    pub fn claim_even(counter: &'a AtomicU32) -> RamVersionClaim<'a> {
+        loop {
+            let current_version = counter.load(Ordering::Acquire);
+            if !current_version.is_multiple_of(2) {
+                continue;
+            }
+
+            // Attempt to increment and therefore successfully claim the version
+            let res = counter.compare_exchange(
+                current_version,
+                current_version.wrapping_add(1),
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            );
+
+            if let Ok(initial_version) = res {
+                return RamVersionClaim {
+                    version_counter: counter,
+                    intial_version: initial_version,
+                };
+            }
+        }
+    }
+
+    /// Reset the state of the version counter to its initial state
+    /// ONLY USE IF YOU ARE SURE RAM HAS NOT BEEN WRITTEN TO
+    pub fn reset(self) {
+        self.version_counter
+            .store(self.intial_version, Ordering::Release);
+        std::mem::forget(self);
+    }
+}
+
+impl<'a> Drop for RamVersionClaim<'a> {
+    fn drop(&mut self) {
+        self.version_counter
+            .store(self.intial_version.wrapping_add(2), Ordering::Release);
+    }
 }
 
 pub const MMIO_SECOND_LEVEL_PAGE_SIZE: usize = 64 * 1024;