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
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