[Qemu-devel] [PATCH] memory: try to inline constant-length reads

[Paolo Bonzini <pbonzini@redhat.com>]

memcpy can take a large amount of time for small reads and writes.
Handle the common case of reading s/g descriptors from memory (there
is no corresponding "write" case that is as common, because writes
often use address_space_st* functions) by inlining the relevant
parts of address_space_read into the caller.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 exec.c                  | 15 ++--------
 include/exec/memory.h   | 74 +++++++++++++++++++++++++++++++++++++------------
 include/exec/ram_addr.h |  1 -
 3 files changed, 59 insertions(+), 31 deletions(-)

diff --git a/exec.c b/exec.c
index 7783012..744e4c7 100644
--- a/exec.c
+++ b/exec.c
@@ -393,17 +393,6 @@ address_space_translate_internal(AddressSpaceDispatch *d, hwaddr addr, hwaddr *x
     return section;
 }
 
-static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
-{
-    if (is_write) {
-        return memory_region_is_ram(mr) && !mr->readonly;
-    } else {
-        return memory_region_is_ram(mr) || memory_region_is_romd(mr);
-    }
-
-    return false;
-}
-
 /* Called from RCU critical section */
 MemoryRegion *address_space_translate(AddressSpace *as, hwaddr addr,
                                       hwaddr *xlat, hwaddr *plen,
@@ -2659,8 +2648,8 @@ MemTxResult address_space_read_continue(AddressSpace *as, hwaddr addr,
     return result;
 }
 
-MemTxResult address_space_read(AddressSpace *as, hwaddr addr, MemTxAttrs attrs,
-                               uint8_t *buf, int len)
+MemTxResult address_space_read_full(AddressSpace *as, hwaddr addr,
+                                    MemTxAttrs attrs, uint8_t *buf, int len)
 {
     hwaddr l;
     hwaddr addr1;
diff --git a/include/exec/memory.h b/include/exec/memory.h
index a0993e5..01f1004 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -1234,23 +1234,7 @@ MemTxResult address_space_write(AddressSpace *as, hwaddr addr,
                                 MemTxAttrs attrs,
                                 const uint8_t *buf, int len);
 
-/**
- * address_space_read: read from an address space.
- *
- * Return a MemTxResult indicating whether the operation succeeded
- * or failed (eg unassigned memory, device rejected the transaction,
- * IOMMU fault).
- *
- * @as: #AddressSpace to be accessed
- * @addr: address within that address space
- * @attrs: memory transaction attributes
- * @buf: buffer with the data transferred
- */
-MemTxResult address_space_read(AddressSpace *as, hwaddr addr, MemTxAttrs attrs,
-                               uint8_t *buf, int len);
-
-/**
- * address_space_ld*: load from an address space
+/* address_space_ld*: load from an address space
  * address_space_st*: store to an address space
  *
  * These functions perform a load or store of the byte, word,
@@ -1385,6 +1369,62 @@ MemTxResult address_space_read_continue(AddressSpace *as, hwaddr addr,
                                         MemTxAttrs attrs, uint8_t *buf,
                                         int len, hwaddr addr1, hwaddr l,
 					MemoryRegion *mr);
+MemTxResult address_space_read_full(AddressSpace *as, hwaddr addr,
+                                    MemTxAttrs attrs, uint8_t *buf, int len);
+void *qemu_get_ram_ptr(ram_addr_t addr);
+
+static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
+{
+    if (is_write) {
+        return memory_region_is_ram(mr) && !mr->readonly;
+    } else {
+        return memory_region_is_ram(mr) || memory_region_is_romd(mr);
+    }
+
+    return false;
+}
+
+/**
+ * address_space_read: read from an address space.
+ *
+ * Return a MemTxResult indicating whether the operation succeeded
+ * or failed (eg unassigned memory, device rejected the transaction,
+ * IOMMU fault).
+ *
+ * @as: #AddressSpace to be accessed
+ * @addr: address within that address space
+ * @attrs: memory transaction attributes
+ * @buf: buffer with the data transferred
+ */
+static inline __attribute__((__always_inline__))
+MemTxResult address_space_read(AddressSpace *as, hwaddr addr, MemTxAttrs attrs,
+                               uint8_t *buf, int len)
+{
+    MemTxResult result = MEMTX_OK;
+    hwaddr l, addr1;
+    void *ptr;
+    MemoryRegion *mr;
+
+    if (__builtin_constant_p(len)) {
+        if (len) {
+            rcu_read_lock();
+            l = len;
+            mr = address_space_translate(as, addr, &addr1, &l, false);
+            if (len == l && memory_access_is_direct(mr, false)) {
+                addr1 += memory_region_get_ram_addr(mr);
+                ptr = qemu_get_ram_ptr(addr1);
+                memcpy(buf, ptr, len);
+            } else {
+                result = address_space_read_continue(as, addr, attrs, buf, len,
+                                                     addr1, l, mr);
+            }
+            rcu_read_unlock();
+        }
+    } else {
+        result = address_space_read_full(as, addr, attrs, buf, len);
+    }
+    return result;
+}
 
 #endif
 
diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index 7115154..7f88eaa 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -73,7 +73,6 @@ ram_addr_t qemu_ram_alloc_resizeable(ram_addr_t size, ram_addr_t max_size,
                                      MemoryRegion *mr, Error **errp);
 int qemu_get_ram_fd(ram_addr_t addr);
 void *qemu_get_ram_block_host_ptr(ram_addr_t addr);
-void *qemu_get_ram_ptr(ram_addr_t addr);
 void qemu_ram_free(ram_addr_t addr);
 void qemu_ram_free_from_ptr(ram_addr_t addr);
 
-- 
2.5.0