Re: [PATCH 0/4] Generic IOMMU page table framework

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Tue, Dec 02, 2014 at 01:47:41PM +0000, Laurent Pinchart wrote:
> On Monday 01 December 2014 12:05:34 Will Deacon wrote:
> > On Sun, Nov 30, 2014 at 10:03:08PM +0000, Laurent Pinchart wrote:
> > > On Thursday 27 November 2014 11:51:14 Will Deacon wrote:
> > > > The LPAE code implements support for 4k/2M/1G, 16k/32M and 64k/512M
> > > > mappings, but I decided not to implement the contiguous bit in the
> > > > interest of trying to keep the code semi-readable. This could always be
> > > > added later, if needed.
> > > 
> > > Do you have any idea how much the contiguous bit can improve performances
> > > in real use cases ?
> > 
> > It depends on the TLB, really. Given that the contiguous sized map directly
> > onto block sizes using different granules, I didn't see that the complexity
> > was worth it.
> > 
> > For example:
> > 
> >    4k granule : 16 contiguous entries => {64k, 32M, 16G}
> >   16k granule : 128 contiguous lvl3 entries => 2M
> >                 32 contiguous lvl2 entries => 1G
> >   64k granule : 32 contiguous entries => {2M, 16G}
> > 
> > If we use block mappings, then we get:
> > 
> >    4k granule : 2M @ lvl2, 1G @ lvl1
> >   16k granule : 32M @ lvl2
> >   64k granule : 512M @ lvl2
> > 
> > so really, we only miss the ability to create 16G mappings.
> 
> In the general case maybe, but as far as I know my IOMMU only supports 4kB 
> granule. Without support for the contiguous bit I loose the ability to create 
> 64kB mappings, which I believe (but haven't test yet) will have a noticeable 
> impact.

It would be good if you could confirm that. I'd have thought that you'd end
up using 2MB mappings most of the time for DMA buffers.

> > I doubt that hardware even implements that size in the TLB (the contiguous
> > bit is only a hint).
> >
> > On top of that, the contiguous bit leads to additional expense on unmap,
> > since you have extra TLB invalidation splitting the thing into non-
> > contiguous pages before you can do anything.
> 
> That will only be required when doing partial unmaps, which shouldn't be that 
> frequent. When unmapping a 64kB block there's no need to split the mapping 
> beforehand.

Sure. I'm not against having support for the contiguous bit, I just don't
plan to implement it myself :)

Will