[Xen-devel] [PATCH] [HVM] Prevent usb driver crashes in Windows

[Ben Guthro <bguthro@xxxxxxxxxxxxxxx>]

qemu-word-tearing.patch:
Use atomic updates to read/write usb controller data.
This can be done because:
    a) word copies on x86 are atomic
    b) The USB spec requires word alignment

This will need to be enhanced once USB 1.2 is supported.

Signed-off-by: Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>

diff -r 86fa3e4277f6 tools/ioemu/target-i386-dm/exec-dm.c
--- a/tools/ioemu/target-i386-dm/exec-dm.c      Mon Jun 04 11:26:30 2007 -0400
+++ b/tools/ioemu/target-i386-dm/exec-dm.c      Mon Jun 04 11:29:03 2007 -0400
@@ -434,6 +434,28 @@ extern unsigned long *logdirty_bitmap;
 extern unsigned long *logdirty_bitmap;
 extern unsigned long logdirty_bitmap_size;
 
+/*
+ * Replace the standard byte memcpy with a int memcpy for appropriately sized
+ * memory copy operations.  Some users (USB-UHCI) can not tolerate the possible
+ * word tearing that can result from a guest concurrently writing a memory
+ * structure while the qemu device model is modifying the same location.
+ * Forcing a int sized read/write prevents the guest from seeing a partially
+ * written int sized atom.
+ */
+void memcpy32(void *dst, void *src, size_t n)
+{
+    if ((n % sizeof(int)) != 0) {
+        memcpy(dst, src, n);
+        return;
+    }
+    n /= sizeof(int);
+    while(n--) {
+        *(int *)dst = *(int *)src;
+        dst += sizeof(int);
+        src += sizeof(int);
+    }
+}
+
 void cpu_physical_memory_rw(target_phys_addr_t addr, uint8_t *buf, 
                             int len, int is_write)
 {
@@ -470,7 +492,7 @@ void cpu_physical_memory_rw(target_phys_
                 }
             } else if ((ptr = phys_ram_addr(addr)) != NULL) {
                 /* Writing to RAM */
-                memcpy(ptr, buf, l);
+                memcpy32(ptr, buf, l);
                 if (logdirty_bitmap != NULL) {
                     /* Record that we have dirtied this frame */
                     unsigned long pfn = addr >> TARGET_PAGE_BITS;
@@ -506,7 +528,7 @@ void cpu_physical_memory_rw(target_phys_
                 }
             } else if ((ptr = phys_ram_addr(addr)) != NULL) {
                 /* Reading from RAM */
-                memcpy(buf, ptr, l);
+                memcpy32(buf, ptr, l);
             } else {
                 /* Neither RAM nor known MMIO space */
                 memset(buf, 0xff, len); 

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