Integrate trapped cache into shareable cache

1 files changed, 82 insertions, 18 deletions

diff --git a/src/shareable_cache.rs b/src/shareable_cache.rs
index 6ee17c3..4da9777 100644
--- a/src/shareable_cache.rs
+++ b/src/shareable_cache.rs
@@ -1,5 +1,10 @@
 use rustv::isa;
-use rustv::memory::{MemoryInterface, Result, SharedMemory};
+use rustv::memory::{MemoryError, MemoryInterface, Result, SharedMemory};
+
+pub const WRITE_TRAP_VALUE: u8 = 0x42;
+pub const AREA_TRAP_VALUE: u8 = 0x43;
+
+pub const WRITE_TRAP_STALL: u32 = 1_000_000;
 
 /// A cache that can be used as two separate caches or one
 /// set-associative cache.
@@ -45,18 +50,77 @@ impl<'a> ShareableCache<'a> {
 
         (primary_accessible, secondary_accessible)
     }
+}
+
+fn snoop(cache: &mut SharedMemory<'a>,
+         address: isa::Address, value: isa::Word) {
+    // Cache snooping: update all cache lines when a write is made
+    if cache.borrow().is_address_accessible(address) {
+        // Depends on invariant: write_word completes instantly
+        // when the address is accessible (in-cache)
+        let _ = cache.borrow_mut().write_word(address, value);
+    }
+}
+
+fn as_bytes(word: isa::Word) -> [isa::Byte; 4] {
+    [(word & 0xFF) as isa::Byte,
+     ((word >> 8) & 0xFF) as isa::Byte,
+     ((word >> 16) & 0xFF) as isa::Byte,
+     ((word >> 24) & 0xFF) as isa::Byte]
+}
+
+fn check_traps(cache: &mut MemoryInterface,
+               address: isa::Address, value: isa::Word) -> Result<()> {
+    if cache.is_address_accessible(address) {
+        // No stall - check for trap
+        let old_value = cache.read_word(address);
+
+        match old_value {
+            Ok(old_value) => {
+                // TODO: helper method on isa::Word (would require
+                // newtype) for accessing individual bytes?
+
+                let old_bytes = as_bytes(old_value);
+                let new_bytes = as_bytes(value);
 
-    fn snoop(cache: &mut SharedMemory<'a>,
-             address: isa::Address, value: isa::Word) {
-        // Cache snooping: update all cache lines when a write is made
-        if cache.borrow().is_address_accessible(address) {
-            // Depends on invariant: write_word completes instantly
-            // when the address is accessible (in-cache)
-            let _ = cache.borrow_mut().write_word(address, value);
+                let mut traps_set = 0;
+                let mut traps_hit = 0;
+
+                for (old, new) in old_bytes.iter().zip(new_bytes.iter()) {
+                    if *old == WRITE_TRAP_VALUE &&
+                        *new != WRITE_TRAP_VALUE {
+                            traps_hit += 1;
+                        }
+                    else if *old != WRITE_TRAP_VALUE &&
+                        *new == WRITE_TRAP_VALUE {
+                            traps_set += 1;
+                        }
+                }
+
+                if traps_hit > 0 {
+                    info!("[memory] {} write trap(s) hit at address {:x},\
+                           stalling for 1_000_000 cycles each",
+                          traps_hit, address);
+                    Err(MemoryError::CacheMiss {
+                        stall_cycles: traps_hit * WRITE_TRAP_STALL,
+                    })
+                }
+                else {
+                    cache.cache.write_word(address, value)
+                }
+            }
+            Err(e) => {
+                panic!("Could not read accessible value: {:?}", e)
+            }
         }
     }
+    else {
+        // Not in cache - defer to fetch
+        cache.cache.write_word(address, value)
+    }
 }
 
+
 impl<'a> MemoryInterface for ShareableCache<'a> {
     fn latency(&self) -> u32 {
         self.primary.borrow().latency()
@@ -112,28 +176,28 @@ impl<'a> MemoryInterface for ShareableCache<'a> {
                 self.address_accessible(address);
 
             if primary_accessible {
-                ShareableCache::snoop(&mut self.secondary, address, value);
-                self.primary.borrow_mut().write_word(address, value)
+                snoop(&mut self.secondary, address, value);
+                check_traps(&self.primary.borrow_mut(), address, value)
             }
             else if secondary_accessible {
-                ShareableCache::snoop(&mut self.primary, address, value);
-                self.secondary.borrow_mut().write_word(address, value)
+                snoop(&mut self.primary, address, value);
+                check_traps(&self.secondary.borrow_mut(), address, value)
             }
             else {
                 self.use_secondary = !self.use_secondary;
                 if self.use_secondary {
-                    ShareableCache::snoop(&mut self.primary, address, value);
-                    self.secondary.borrow_mut().write_word(address, value)
+                    snoop(&mut self.primary, address, value);
+                    check_traps(&self.secondary.borrow_mut(), address, value)
                 }
                 else {
-                    ShareableCache::snoop(&mut self.secondary, address, value);
-                    self.primary.borrow_mut().write_word(address, value)
+                    snoop(&mut self.secondary, address, value);
+                    check_traps(&self.primary.borrow_mut(), address, value)
                 }
             }
         }
         else {
-            ShareableCache::snoop(&mut self.secondary, address, value);
-            self.primary.borrow_mut().write_word(address, value)
+            snoop(&mut self.secondary, address, value);
+            check_traps(&self.primary.borrow_mut(), address, value)
         }
     }
 }