Re: [Qemu-devel] [PATCH v2 1/2] xbzrle: don't check the value in the vm ram repeatedly

[Paolo Bonzini]

Il 31/03/2014 16:00, Dr. David Alan Gilbert ha scritto:

* arei.gong...@huawei.com (arei.gong...@huawei.com) wrote:

From: ChenLiang <chenlian...@huawei.com>

xbzrle_encode_buffer checks the value in the vm ram repeatedly.
It is risk if runs xbzrle_encode_buffer on changing data.
And it is not necessary.

Reported-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
Signed-off-by: ChenLiang <chenlian...@huawei.com>
Signed-off-by: Gonglei <arei.gong...@huawei.com>
---
 xbzrle.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/xbzrle.c b/xbzrle.c
index fbcb35d..bf08c56 100644
--- a/xbzrle.c
+++ b/xbzrle.c
@@ -27,7 +27,7 @@ int xbzrle_encode_buffer(uint8_t *old_buf, uint8_t *new_buf, 
int slen,
                          uint8_t *dst, int dlen)
 {
     uint32_t zrun_len = 0, nzrun_len = 0;
-    int d = 0, i = 0;
+    int d = 0, i = 0, j;
     long res, xor;
     uint8_t *nzrun_start = NULL;

@@ -82,6 +82,8 @@ int xbzrle_encode_buffer(uint8_t *old_buf, uint8_t *new_buf, 
int slen,
         if (d + 2 > dlen) {
             return -1;
         }
+        i++;
+        nzrun_len++;
         /* not aligned to sizeof(long) */
         res = (slen - i) % sizeof(long);
         while (res && old_buf[i] != new_buf[i]) {
@@ -98,11 +100,17 @@ int xbzrle_encode_buffer(uint8_t *old_buf, uint8_t 
*new_buf, int slen,
                 xor = *(long *)(old_buf + i) ^ *(long *)(new_buf + i);
                 if ((xor - mask) & ~xor & (mask << 7)) {
                     /* found the end of an nzrun within the current long */
-                    while (old_buf[i] != new_buf[i]) {
-                        nzrun_len++;
-                        i++;
+                    for (j = 0; j < sizeof(long); j++) {
+                        if (old_buf[i] != new_buf[i]) {
+                            nzrun_len++;
+                            i++;
+                        } else {
+                            break;
+                        }
+                    }
+                    if (j != sizeof(long)) {
+                        break;

I wonder if it would be easier just to use the value of 'xor' - since that
already contains the value that we read, and if we've got this far is guaranteed
to have a none-equal byte in it.  That would be something like (untested):

  for (j = 0; j < sizeof(long); j++) {
      if (get_byte(xor, j) != 0) {
          break;
      }
  }
  nzrun_len += j;
  i += j;

Or this:

#ifdef HOST_WORDS_BIGENDIAN
    j = clzl(xor) >> 3;
#else
    j = ctzl(xor) >> 3;
#endif
    nzrun_len += j;
    i += j;

Paolo