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

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 11-Oct-18 11:39 AM, Shreyansh Jain wrote:
> On Thursday 11 October 2018 03:43 PM, Burakov, Anatoly wrote:
>> On 11-Oct-18 11:07 AM, Shreyansh Jain wrote:
>>> 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.
>>
>> Mem events are not triggered on rte_free(), they're triggered on page 
>> deallocation. A call to rte_free/rte_memzone_free/rte_mempool_free 
>> etc. *might* trigger a page deallocation, but *only* if the memory 
>> area being freed encompasses an entire page. If you rte_malloc() 64 
>> bytes and then rte_free() those 64 bytes, you won't get a mem event 
>> *unless* these were the only 64 bytes allocated on a particular page, 
>> and the entire page is no longer used by anything else.
> 
> My understanding is same.
> But, it seems my explanation wasn't well written:
> 
> For a rte_free(), I am not expecting that mem_event is raised - but, the 
> caller of rte_free() (the eth or crypto drivers, or applications) may 
> call the PA-VA table del function to remove the entries.
> 
> This voluntary delete of table entry from the drivers or applications 
> using PA-VA calling del of PA-VA table - is not correct.

Yes, so it appears :)

-- 
Thanks,
Anatoly