[PATCH 09/16] dma-direct: Support PCI P2PDMA pages in dma-direct map_sg

[Logan Gunthorpe]

Oops missed a comment:

On 2021-05-02 5:28 p.m., John Hubbard wrote:
>>   int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
>>   		enum dma_data_direction dir, unsigned long attrs)
>>   {
>> -	int i;
>> +	struct pci_p2pdma_map_state p2pdma_state = {};
> 
> Is it worth putting this stuff on the stack--is there a noticeable
> performance improvement from caching the state? Because if it's
> invisible, then simplicity is better. I suspect you're right, and that
> it *is* worth it, but it's good to know for real.

I haven't measured it (it would be hard to measure), but I think it's
fairly clear here. Without the state, xa_load() would need to be called
on *every* page in an SGL that maps only P2PDMA memory from one device.
With the state, it only needs to be called once. xa_load() is cheap, but
it is not that cheap.

There's essentially the same optimization in get_user_pages for
ZONE_DEVICE pages. So, if it is necessary there, it should be necessary
here.

Logan