[PATCH v11 09/13] copy-on-read: skip non-guest reads if no copy needed

[Andrey Shinkevich]

On 14.10.2020 15:51, Max Reitz wrote:
> On 12.10.20 19:43, Andrey Shinkevich wrote:
>> If the flag BDRV_REQ_PREFETCH was set, pass it further to the
>> COR-driver to skip unneeded reading. It can be taken into account for
>> the COR-algorithms optimization. That check is being made during the
>> block stream job by the moment.
>>
>> Signed-off-by: Andrey Shinkevich <andrey.shinkevich at virtuozzo.com>
>> ---
>>   block/copy-on-read.c | 13 +++++++++----
>>   block/io.c           |  3 ++-
>>   2 files changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/block/copy-on-read.c b/block/copy-on-read.c
>> index b136895..278a11a 100644
>> --- a/block/copy-on-read.c
>> +++ b/block/copy-on-read.c
>> @@ -148,10 +148,15 @@ static int coroutine_fn cor_co_preadv_part(BlockDriverState *bs,
>>               }
>>           }
>>   
>> -        ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
>> -                                  local_flags);
>> -        if (ret < 0) {
>> -            return ret;
>> +        if (!!(flags & BDRV_REQ_PREFETCH) &
> 
> How about dropping the double negation and using a logical && instead of
> the binary &?
> 

Yes, that's correct.

>> +            !(local_flags & BDRV_REQ_COPY_ON_READ)) {
>> +            /* Skip non-guest reads if no copy needed */
>> +        } else {
> 
> Hm.  I would have just written the negated form
> 
> (!(flags & BDRV_REQ_PREFETCH) || (local_flags & BDRV_REQ_COPY_ON_READ))
> 
> and put the “skip” comment above that condition.
> 
> (Since local_flags is initialized to flags, it can be written as a
> single comparison, but that’s a matter of taste and I’m not going to
> recommend either over the other:
> 

I played with the flags to make the idea obvious for the eye of a 
beholder: "we neither read nor write".
Comparing the BDRV_REQ_PREFETCH against the 'flags' means that the flag 
comes from outside of the function.
And the empty section means we do nothing in that case.
Eventually, I will pick up the brief expression below.
Thanks,

Andrey

> ((local_flags & (BDRV_REQ_PREFETCH | BDRV_REQ_COPY_ON_READ)) !=
> BDRV_REQ_PREFETCH)
> 
> )
> 
>> +            ret = bdrv_co_preadv_part(bs->file, offset, n, qiov, qiov_offset,
>> +                                      local_flags);
>> +            if (ret < 0) {
>> +                return ret;
>> +            }
>>           }
>>   
>>           offset += n;
>> diff --git a/block/io.c b/block/io.c
>> index 11df188..bff1808 100644
>> --- a/block/io.c
>> +++ b/block/io.c
>> @@ -1512,7 +1512,8 @@ static int coroutine_fn bdrv_aligned_preadv(BdrvChild *child,
>>   
>>       max_bytes = ROUND_UP(MAX(0, total_bytes - offset), align);
>>       if (bytes <= max_bytes && bytes <= max_transfer) {
>> -        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset, 0);
>> +        ret = bdrv_driver_preadv(bs, offset, bytes, qiov, qiov_offset,
>> +                                 flags & bs->supported_read_flags);
> 
> Ah, OK.  I see.  I expected this to be a separate patch.  I still wonder
> why it isn’t.
> 
> Max
> 
>>           goto out;
>>       }
>>   
>>
> 
>