Re: [PATCH v1 2/2] dma-mapping-common: add DMA attribute - DMA_ATTR_IOMMU_BYPASS

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2015-11-02 at 14:07 +0200, Shamir Rabinovitch wrote:
> On Mon, Nov 02, 2015 at 09:00:34PM +1100, Benjamin Herrenschmidt
> wrote:
> > 
> > Chosing on a per-mapping basis *in the back end* might still make
> > some
> 
> In my case, choosing mapping based on the hardware that will use this
> mappings makes more sense. Most hardware are not that performance 
> sensitive as the Infiniband hardware.

 ...

> The driver know for what hardware it is mapping the memory so it know 
> if the memory will be used by performance sensitive hardware or not.

Then I would argue for naming this differently. Make it an optional
hint "DMA_ATTR_HIGH_PERF" or something like that. Whether this is
achieved via using a bypass or other means in the backend not the
business of the driver.

> In your case, what will give the better performance - 1:1 mapping or
> IOMMU
> mapping? When you say 'relaxing the protection' you refer to 1:1
> mapping?

> Also, how this 1:1 window address the security concerns that other
> raised
> by other here?

It will partially only but it's just an example of another way the
bakcend could provide some improved performances without a bypass.

Cheers,
Ben.