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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 31/03/2014 16:00, Dr. David Alan Gilbert ha scritto:
> * arei.gonglei@huawei.com (arei.gonglei@huawei.com) wrote:
>> From: ChenLiang <chenliang88@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 <dgilbert@redhat.com>
>> Signed-off-by: ChenLiang <chenliang88@huawei.com>
>> Signed-off-by: Gonglei <arei.gonglei@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