Re: [dpdk-dev] [EXT] Re: [PATCH] ethdev: fix DMA zone reserve not honoring size

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 4/2/19 11:25 AM, Jerin Jacob Kollanukkaran wrote:
> On Tue, 2019-04-02 at 10:36 +0300, Andrew Rybchenko wrote:
>> On 4/2/19 3:47 AM, Jerin Jacob Kollanukkaran wrote:
>>> On Mon, 2019-04-01 at 10:30 +0300, Andrew Rybchenko wrote:
>>>> External Email
>>>> On 3/31/19 7:25 PM, Pavan Nikhilesh Bhagavatula wrote:
>>>>> From: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>>>
>>>>> The `rte_eth_dma_zone_reserve()` is generally used to create HW
>>>>> rings.
>>>>> In some scenarios when a driver needs to reconfigure the ring
>>>>> size
>>>>> since the named memzone already exists it returns the previous
>>>>> memzone
>>>>> without checking if a different sized ring is requested.
>>>>>
>>>>> Introduce a check to see if the ring size requested is
>>>>> different
>>>>> from the
>>>>> previously created memzone length.
>>>>>
>>>>> Fixes: 719dbebceb81 ("xen: allow determining DOM0 at runtime")
>>>>> Cc: stable@dpdk.org
>>>>>
>>>>> Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
>>>>> ---
>>>>>   lib/librte_ethdev/rte_ethdev.c | 5 ++++-
>>>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/lib/librte_ethdev/rte_ethdev.c
>>>>> b/lib/librte_ethdev/rte_ethdev.c
>>>>> index 12b66b68c..4ae12e43b 100644
>>>>> --- a/lib/librte_ethdev/rte_ethdev.c
>>>>> +++ b/lib/librte_ethdev/rte_ethdev.c
>>>>> @@ -3604,9 +3604,12 @@ rte_eth_dma_zone_reserve(const struct
>>>>> rte_eth_dev *dev, const char *ring_name,
>>>>>   	}
>>>>>   
>>>>>   	mz = rte_memzone_lookup(z_name);
>>>>> -	if (mz)
>>>>> +	if (mz && (mz->len == size))
>>>>>   		return mz;
>>>>>   
>>>>> +	if (mz)
>>>>> +		rte_memzone_free(mz);
>>>>   
>>>> NACK
>>>> I really don't like that API which should reserve does free if
>>>> requested
>>>> size does not match previously allocated.
>>> Why? Is due to API name?
>>   
>> 1. The problem really exists. The problem is bad and it very good
>> that you
>>      caught it and came up with a patch. Many thanks.
>> 2. Silently free and reallocate memory is bad. Memory could be
>> used/mapped etc.
> If I understand it correctly, Its been used while configuring
> the device and it is per queue, If so, Is there any case where
> memory in use in parallel in real world case with DPDK?

"in real world case with DPDK" is very fragile justification.
I simply don't want to dig in this way since it is very easy to make
a mistake or simply false assumption.

>> 3. As an absolute minimum if we accept the behaviour it must be
>> documented
>>      in the function description.
>>
>>>   If so,
>>> Can we have rte_eth_dma_zone_reservere_with_resize() then ?
>>> or any another name, You would like to have?
>>   
>> 4. I'd prefer an error if different size (or bigger) memzone is
>> requested,
>>      but I understand that it can break existing drivers.
>>
>> Thomas, Ferruh, what do you think?
>>
>>>> I understand the motivation, but I don't think the solution is
>>>> correct.
>>> What you think it has correct solution then?
>>   
>> See above plus handling in drivers or dedicated function with
>> better name as you suggest above.
> Handling in driver means return error?

Yes.

> Regarding API, Yes, We can add new API. What we will do that exiting
> driver. Is up to driver maintainers to use the new API. I am fine with
> either approach, Just asking the opinion.

You have mine, but I'd like to know what other ethdev maintainers
think about it.

>>> Obviously, We can not allocate max ring size in init time.
>>> If the NIC has support for 64K HW ring, We will be wasting too much
>>> as
>>> it is per queue.
>>   
>> Yes, I agree that it is an overkill.
>>
>> net/sfc tries to carefully free/reserve on NIC/queues reconfigure.
>>
>> Many thanks,
>> Andrew.