sim: Fix a faulty assumption in MemPool.

In the MemPool object, the idea of a limit of the pool (largest page)
and the total number of pages were conflated, as was the page number of
the next "free" page and the total number of pages allocated. Both of
those would only be equivalent if the memory pool starts at address
zero, which is not generally true and could be true for at most one pool
at a time even when it is occasionally true.

Instead, this change fixes up MemPool to keep tree values, a starting
page number, the page number of the next free page, and the total number
of pages in the pool, both allocated and unallocated.

With those three values, we can accurately report the number of
allocated pages (not just the number of pages of any kind below the next
free one), the total number of free pages, and the total number of pages
in general (not the largest numbered page in the pool).

The value serialized by the System class was adjusted so that it will
stay compatible with previous checkpoints. The value unserialized by the
system class is passed to the MemPool as a limit, which has not changed
and so doesn't need to be updated. It gets translated into the total
number of pages in the MemPool constructor.

Change-Id: I8268ef410b41bf757df9ee5585ec2f6b0d8499e1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50687
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/mem_pool.cc

@@ -39,9 +39,17 @@ namespace gem5
 {
 
 MemPool::MemPool(Addr page_shift, Addr ptr, Addr limit)
-        : pageShift(page_shift), freePageNum(ptr >> page_shift),
-        _totalPages(limit >> page_shift)
+        : pageShift(page_shift), startPageNum(ptr >> page_shift),
+        freePageNum(ptr >> page_shift),
+        _totalPages((limit - ptr) >> page_shift)
 {
+    gem5_assert(_totalPages > 0);
+}
+
+Counter
+MemPool::startPage() const
+{
+    return startPageNum;
 }
 
 Counter
@@ -71,13 +79,19 @@ MemPool::totalPages() const
 Counter
 MemPool::allocatedPages() const
 {
-    return freePageNum;
+    return freePageNum - startPageNum;
 }
 
 Counter
 MemPool::freePages() const
 {
-    return _totalPages - freePageNum;
+    return _totalPages - allocatedPages();
+}
+
+Addr
+MemPool::startAddr() const
+{
+    return startPage() << pageShift;
 }
 
 Addr

src/sim/mem_pool.hh

@@ -47,7 +47,10 @@ class MemPool
   private:
     Addr pageShift;
 
-    /** Page number to free memory. */
+    /** Start page of pool. */
+    Counter startPageNum;
+
+    /** Page number of free memory. */
     Counter freePageNum;
 
     /** The size of the pool, in number of pages. */
@@ -56,6 +59,7 @@ class MemPool
   public:
     MemPool(Addr page_shift, Addr ptr, Addr limit);
 
+    Counter startPage() const;
     Counter freePage() const;
     void setFreePage(Counter value);
     Addr freePageAddr() const;
@@ -64,6 +68,7 @@ class MemPool
     Counter allocatedPages() const;
     Counter freePages() const;
 
+    Addr startAddr() const;
     Addr allocatedBytes() const;
     Addr freeBytes() const;
     Addr totalBytes() const;

src/sim/system.cc

@@ -435,7 +435,7 @@ System::serialize(CheckpointOut &cp) const
 
     for (const auto& memPool : memPools) {
         ptrs.push_back(memPool.freePageAddr());
-        limits.push_back(memPool.totalBytes());
+        limits.push_back(memPool.totalBytes() + memPool.startAddr());
     }
 
     SERIALIZE_CONTAINER(ptrs);