[Xen-devel] [PATCH] Re: [Xen-staging] [xen-unstable] [HVM] Prevent usb driver crashes in Windows

[Alex Williamson <alex.williamson@xxxxxx>]

On Wed, 2007-06-06 at 20:26 +0100, Xen staging patchbot-unstable wrote: 
> +/*
> + * Replace the standard byte memcpy with a word 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 word-sized read/write prevents the guest from seeing a partially
> + * written word-sized atom.
> + */
> +void memcpy_words(void *dst, void *src, size_t n)
> +{
> +    while (n >= sizeof(long)) {
> +        *((long *)dst)++ = *((long *)src)++;
> +        n -= sizeof(long);
> +    }

The above requires 8-byte alignment on 64bit boxes, not the 4-byte
alignment USB provides.  This generates lots of unaligned accesses on
ia64 and probably slows things down on x64 as well.  How about the patch
below?  Thanks,

        Alex

Signed-off-by: Alex Williamson <alex.williamson@xxxxxx>
---

diff -r 7d4c40c21690 tools/ioemu/target-i386-dm/exec-dm.c
--- a/tools/ioemu/target-i386-dm/exec-dm.c      Mon Jun 18 13:50:42 2007 -0600
+++ b/tools/ioemu/target-i386-dm/exec-dm.c      Mon Jun 18 15:43:27 2007 -0600
@@ -445,11 +445,11 @@ extern unsigned long logdirty_bitmap_siz
  */
 void memcpy_words(void *dst, void *src, size_t n)
 {
-    while (n >= sizeof(long)) {
-        *((long *)dst) = *((long *)src);
-        dst = ((long *)dst) + 1;
-        src = ((long *)src) + 1;
-        n -= sizeof(long);
+    while (n > sizeof(int)) {
+        *((int *)dst) = *((int *)src);
+        dst = ((int *)dst) + 1;
+        src = ((int *)src) + 1;
+        n -= sizeof(int);
     }
 
     if (n & 4) {



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