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 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.

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.

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