Re: [RFC RESEND 00/16] Split IOMMU DMA mapping operation to two steps

[Christoph Hellwig]

On Wed, Mar 06, 2024 at 11:43:28AM -0400, Jason Gunthorpe wrote:
> I don't think they are so fundamentally different, at least in our
> past conversations I never came out with the idea we should burden the
> driver with two different flows based on what kind of alignment the
> transfer happens to have.

Then we talked past each other..

> At least the RDMA drivers could productively use just a page aligned
> interface. But I didn't think this would make BIO users happy so never
> even thought about it..

page aligned is generally the right thing for the block layer.  NVMe
for example already requires that anyway due to PRPs.

> > The total transfer size should just be passed in by the callers and
> > be known, and there should be no offset.
> 
> The API needs the caller to figure out the total number of IOVA pages
> it needs, rounding up the CPU ranges to full aligned pages. That
> becomes the IOVA allocation.

Yes, it's a basic align up to the granularity asuming we don't bother
with non-aligned transfers.

> 
> > So if we want to efficiently be able to handle these cases we need
> > two APIs in the driver and a good framework to switch between them.
> 
> But, what does the non-page-aligned version look like? Doesn't it
> still look basically like this?

I'd just rather have the non-aligned case for those who really need
it be the loop over map single region that is needed for the direct
mapping anyway.

> 
> And what is the actual difference if the input is aligned? The caller
> can assume it doesn't need to provide a per-range dma_addr_t during
> unmap.

A per-range dma_addr_t doesn't really make sense for the aligned and
coalesced case.

> It still can't assume the HW programming will be linear due to the P2P
> !ACS support.
> 
> And it still has to call an API per-cpu range to actually program the
> IOMMU.
> 
> So are they really so different to want different APIs? That strikes
> me as a big driver cost.

To not have to store a dma_address range per CPU range that doesn't
actually get used at all.