Re: [dpdk-dev] [PATCH 3/5] common/dpaax: add library for PA VA translation table

[Shreyansh Jain <shreyansh.jain@nxp.com>]

On Thursday 11 October 2018 03:32 PM, Shreyansh Jain wrote:
> On Thursday 11 October 2018 02:33 PM, Burakov, Anatoly wrote:
>> On 09-Oct-18 11:45 AM, Shreyansh Jain wrote:
>>> On Tuesday 25 September 2018 07:09 PM, Shreyansh Jain wrote:
>>>> Hello Anatoly,
>>>>
>>>> On Tuesday 25 September 2018 06:58 PM, Burakov, Anatoly wrote:
>>>>> On 25-Sep-18 1:54 PM, Shreyansh Jain wrote:
>>>>>> A common library, valid for dpaaX drivers, which is used to maintain
>>>>>> a local copy of PA->VA translations.
>>>>>>
>>>>>> In case of physical addressing mode (one of the option for FSLMC, and
>>>>>> only option for DPAA bus), the addresses of descriptors Rx'd are
>>>>>> physical. These need to be converted into equivalent VA for rte_mbuf
>>>>>> and other similar calls.
>>>>>>
>>>>>> Using the rte_mem_virt2iova or rte_mem_virt2phy is expensive. This
>>>>>> library is an attempt to reduce the overall cost associated with
>>>>>> this translation.
>>>>>>
>>>>>> A small table is maintained, containing continuous entries
>>>>>> representing a continguous physical range. Each of these entries
>>>>>> stores the equivalent VA, which is fed during mempool creation, or
>>>>>> memory allocation/deallocation callbacks.
>>>>>>
>>>
>>> [...]
>>>
>>>>
>>>>>
>>>>> Also, a couple of nitpicks below.
>>>>>
>>>>>>   cosnfig/common_base                            |   5 +
>>>>>>   config/common_linuxapp                        |   5 +
>>>>>>   drivers/common/Makefile                       |   4 +
>>>>>>   drivers/common/dpaax/Makefile                 |  31 ++
>>>>>>   drivers/common/dpaax/dpaax_iova_table.c       | 509 
>>>>>> ++++++++++++++++++
>>>>>>   drivers/common/dpaax/dpaax_iova_table.h       | 104 ++++
>>>>>>   drivers/common/dpaax/dpaax_logs.h             |  39 ++
>>>>>>   drivers/common/dpaax/meson.build              |  12 +
>>>>>
>>>>> <snip>
>>>>>
>>>>>> +    DPAAX_DEBUG("Add: Found slot at (%"PRIu64")[(%zu)] for 
>>>>>> vaddr:(%p),"
>>>>>> +            " phy(%"PRIu64"), len(%zu)", entry[i].start, e_offset,
>>>>>> +            vaddr, paddr, length);
>>>>>> +    return 0;
>>>>>> +}
>>>>>> +
>>>>>> +int
>>>>>> +dpaax_iova_table_del(phys_addr_t paddr, size_t len __rte_unused)
>>>>>
>>>>> len is not unused.
>>>>
>>>> I will fix this.
>>>> Actually, this function itself is useless - more for symmetry reason.
>>>> Callers would be either simply updating the table, or ignoring it 
>>>> completely. But, yes, this is indeed wrong that I set that unused.
>>>>
>>>
>>> Actually, I was wrong in my first reply. In case of 
>>> dpaax_iova_table_del(), len is indeed redundant. This is because the 
>>> mapping is for a complete page (min of 2MB size), even if the request 
>>> is for lesser length. So, removal of a single entry (of fixed size) 
>>> would be done.
>>>
>>> In fact, while on this, I think deleting a PA->VA entry itself is 
>>> incorrect (not just useless). A single entry (~2MB equivalent) can 
>>> represent multiple users (working on a rte_malloc'd area, for 
>>> example). So, effectively, its always an update - not an add or del.
>>
>> I'm not sure what you mean here. If you got a mem event about memory 
>> area being freed, it's guaranteed to *not* have any users - neither 
>> malloc, nor any other memory. And len is always page-aligned.
> 
> ok. Maybe I am getting this wrong, but consider this:
> 
> 1) hugepage size=2MB
> 2) a = malloc(1M)
>    this will pin an entry in table for a block starting at VA=(a) and 
> PA=(a'). Each entry is of 2MB length - that means, even if someone were 
> to access a+1048577 for an equivalent PA, they would get it (though, 
> that is a incorrect access).
> 3) b = malloc(1M)
>    this *might* lead to a case where same 2MB page is used and 
> VA=(b==(a+1MB)). Being hugepage backed, PA=(b=PA(a)+1M).
> = After b, the PA-VA table has a single entry of 2MB, representing two 
> mallocs. It can be used for translation for any thread requesting PAs of 
> a or b.
> 4) Free(a)
>   - this would attempt to remove one 2MB entry from PA-VA table. But, 
> 'b' is already valid. Access to get_pa(VA(b)) should return me the PA(b).
>   - 'len' is not even used as the entry in PA-VA table is of a fixed size.

Just to add to this:
- if talking about the mem_event callback, it definitely won't be a case 
where same page is still being served under another rte_malloc
- But, calls can come to delete from users of PA-VA table based on their 
own rte_free().

And, your comment makes me think - I should probably del entry from the 
table only when mem_event callback is received.

> 
> In the above, (3) is an assumption I am making based on my understanding 
> how mem allocator is working. Is that wrong?
> 
> Basically, this is a restriction of this table - it has a min chunk of 
> 2MB - even for 1G hugepages - and hence, it is not possible to honor 
> deletes. I know this is convoluted logic - but, this keeps it simple and 
> use-able without much performance impact.
> 
> [...]
>