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

[Andrey Shinkevich]

On 14.10.2020 18:22, Vladimir Sementsov-Ogievskiy wrote:
> 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 &?
>>
>>> +            !(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:
>>
>> ((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);
> 
> 
> When BDRV_REQ_PREFETCH is passed, qiov may be (and generally should be) 
> NULL. This means, that we can't just drop the flag when call the driver 
> that doesn't support it.
> 
> Actually, if driver doesn't support the PREFETCH flag we should do nothing.
> 
> 
>>
>> Ah, OK.  I see.  I expected this to be a separate patch.  I still wonder
>> why it isn’t.
>>
> 
> 
> Could it be part of patch 07? I mean introduce new field 
> supported_read_flags and handle it in generic code in one patch, prior 
> to implementing support for it in COR driver.
> 
> 

We have to add the supported flags for the COR driver in the same patch. 
Or before handling the supported_read_flags at the generic layer 
(handling zero does not make a sence). Otherwise, the test #216 (where 
the COR-filter is applied) will not pass.

Andrey