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Date: Fri, 13 Sep 2013 10:25:07 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PULL 33/42] block: return BDRV_BLOCK_ZERO past
 end of backing file
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To: Peter Lieven <lieven-lists@dlhnet.de>
Cc: Anthony Liguori <anthony@codemonkey.ws>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

Il 13/09/2013 09:33, Peter Lieven ha scritto:
> On 06.09.2013 17:39, Stefan Hajnoczi wrote:
>> From: Paolo Bonzini <pbonzini@redhat.com>
>>
>> If the sectors are unallocated and we are past the end of the
>> backing file, they will read as zero.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
>> ---
>>   block.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index aa9ec83..82bbd6c 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -3102,8 +3102,16 @@ static int64_t coroutine_fn
>> bdrv_co_get_block_status(BlockDriverState *bs,
>>           return ret;
>>       }
>>   -    if (!(ret & BDRV_BLOCK_DATA) && bdrv_has_zero_init(bs)) {
>> -        ret |= BDRV_BLOCK_ZERO;
>> +    if (!(ret & BDRV_BLOCK_DATA)) {
>> +        if (bdrv_has_zero_init(bs)) {
> this should be bdi->discard_zeroes. bdrv_has_zero_init() does only give
> a valid result
> right after bdrv_create(). i currently working on extending bdi. I can
> send a patch for this
> if you agree.

Yes, please.  Right now there is no bdi->discard_zeroes.

>> +            ret |= BDRV_BLOCK_ZERO;
>> +        } else {
>> +            BlockDriverState *bs2 = bs->backing_hd;
> 
> this segfaults if there is no backing_hd. found while testing
> get_block_status with iscsi.
> 
> paolo, is this the correct fix?

Yes, thanks.

Paolo

> @@ -3110,7 +3157,7 @@ static int64_t coroutine_fn
> bdrv_co_get_block_status(BlockDriverState *bs,
>      if (!(ret & BDRV_BLOCK_DATA)) {
>          if (bdrv_has_zero_init(bs)) {
>              ret |= BDRV_BLOCK_ZERO;
> -        } else {
> +        } else if (bs->backing_hd)  {
>              BlockDriverState *bs2 = bs->backing_hd;
>              int64_t length2 = bdrv_getlength(bs2);
>              if (length2 >= 0 && sector_num >= (length2 >>
> BDRV_SECTOR_BITS)) {
> 
> 
>> +            int64_t length2 = bdrv_getlength(bs2);
>> +            if (length2 >= 0 && sector_num >= (length2 >>
>> BDRV_SECTOR_BITS)) {
>> +                ret |= BDRV_BLOCK_ZERO;
>> +            }
>> +        }
>>       }
>>       return ret;
>>   }
>