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

[Andrey Shinkevich]

On 21.10.2020 23:43, Andrey Shinkevich wrote:
> 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>
>>>> ---

[...]

>>>> 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

I have found a workaround and am going to send all the related patches 
as a separate series.

Andrey