Re: [RFC rdma 1/3] RDMA/core: Create a common mmap function

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Jul 03, 2019 at 11:19:34AM +0300, Gal Pressman wrote:
> On 03/07/2019 1:31, Jason Gunthorpe wrote:
> >> Seems except Mellanox + hns the mmap flags aren't ABI. 
> >> Also, current Mellanox code seems like it won't benefit from 
> >> mmap cookie helper functions in any case as the mmap function is very specific and the flags used indicate 
> >> the address and not just how to map it.
> > 
> > IMHO, mlx5 has a goofy implementaiton here as it codes all of the object
> > type, handle and cachability flags in one thing.
> 
> Do we need object type flags as well in the generic mmap code?

At the end of the day the driver needs to know what page to map during
the mmap syscall.

mlx5 does this by encoding the page type in the address, and then many
types have seperate lookups based onthe offset for the actual page.

IMHO the single lookup and opaque offset is generally better..

Since the mlx5 scheme is ABI it can't be changed unfortunately.

If you want to do user controlled cachability flags, or not, is a fair
question, but they still become ABI..

I'm wondering if it really makes sense to do that during the mmap, or
if the cachability should be set as part of creating the cookie?

> Another issue is that these flags aren't exposed in an ABI file, so
> a userspace library can't really make use of it in current state.

Woops.

Ah, this is all ABI so you need to dig out of this hole ASAP :)

Jason