netdev - Re: [PATCH net-next 2/3] net: page_pool: add the possibility to sync DMA memory for non-coherent devices

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Date:   Thu, 14 Nov 2019 13:04:26 -0800
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Ilias Apalodimas" <ilias.apalodimas@...aro.org>
Cc:     "Lorenzo Bianconi" <lorenzo@...nel.org>, netdev@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net,
        thomas.petazzoni@...tlin.com, brouer@...hat.com,
        matteo.croce@...hat.com
Subject: Re: [PATCH net-next 2/3] net: page_pool: add the possibility to sync
 DMA memory for non-coherent devices

On 14 Nov 2019, at 12:42, Ilias Apalodimas wrote:

> Hi Jonathan,
>
> On Thu, Nov 14, 2019 at 12:27:40PM -0800, Jonathan Lemon wrote:
>>
>>
>> On 14 Nov 2019, at 10:53, Ilias Apalodimas wrote:
>>
>>> [...]
>>>>> index 2cbcdbdec254..defbfd90ab46 100644
>>>>> --- a/include/net/page_pool.h
>>>>> +++ b/include/net/page_pool.h
>>>>> @@ -65,6 +65,9 @@ struct page_pool_params {
>>>>>  	int		nid;  /* Numa node id to allocate from pages from */
>>>>>  	struct device	*dev; /* device, for DMA pre-mapping purposes */
>>>>>  	enum dma_data_direction dma_dir; /* DMA mapping direction */
>>>>> +	unsigned int	max_len; /* max DMA sync memory size */
>>>>> +	unsigned int	offset;  /* DMA addr offset */
>>>>> +	u8 sync;
>>>>>  };
>>>>
>>>> How about using PP_FLAG_DMA_SYNC instead of another flag word?
>>>> (then it can also be gated on having DMA_MAP enabled)
>>>
>>> You mean instead of the u8?
>>> As you pointed out on your V2 comment of the mail, some cards don't 
>>> sync
>>> back to
>>> device.
>>> As the API tries to be generic a u8 was choosen instead of a flag to
>>> cover these
>>> use cases. So in time we'll change the semantics of this to 'always
>>> sync', 'dont
>>> sync if it's an skb-only queue' etc.
>>> The first case Lorenzo covered is sync the required len only instead 
>>> of
>>> the full
>>> buffer
>>
>> Yes, I meant instead of:
>> +		.sync = 1,
>>
>> Something like:
>>         .flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC
>>
>> Since .sync alone doesn't make sense if the page pool isn't 
>> performing any
>> DMA mapping, right?
>
> Correct. If the sync happens regardless of the page pool mapping 
> capabilities,
> this will affect performance negatively as well (on non-coherent 
> architectures)
>
>> Then existing drivers, if they're converted, can just
>> add the SYNC flag.
>>
>> I did see the initial case where only the RX_BUF_SIZE (1536) is 
>> sync'd
>> instead of the full page.
>>
>> Could you expand on your 'skb-only queue' comment?  I'm currently 
>> running
>> a variant of your patch where iommu mapped pages are attached to 
>> skb's and
>> sent up the stack, then reclaimed on release.  I imagine that with 
>> this
>> change, they would have the full RX_BUF_SIZE sync'd before returning 
>> to the
>> driver, since the upper layers could basically do anything with the 
>> buffer
>> area.
>
> The idea was that page_pool lives per device queue. Usually some 
> queues are
> reserved for XDP only. Since eBPF progs can change the packet we have 
> to sync
> for the device, before we fill in the device descriptors.

And some devices (mlx4) run xdp on the normal RX queue, and if the 
verdict is
PASS, a skb is constructed and sent up the stack.


> For the skb reserved queues, this depends on the 'anything'. If the 
> rest of the
> layers touch (or rather write) into that area, then we'll again gave 
> to sync.
> If we know that the data has not been altered though, we can hand them 
> back to
> the device skipping that sync right?

Sure, but this is also true for eBPF programs.  How would the driver 
know that
the data has not been altered / compacted by the upper layers?
-- 
Jonathan