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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Jul 05, 2019 at 05:24:18PM +0000, Michal Kalderon wrote:
> > From: Jason Gunthorpe <jgg@ziepe.ca>
> > Sent: Friday, July 5, 2019 6:33 PM
> > 
> > On Fri, Jul 05, 2019 at 03:29:03PM +0000, Michal Kalderon wrote:
> > > > From: Jason Gunthorpe <jgg@ziepe.ca>
> > > > Sent: Thursday, July 4, 2019 3:35 PM
> > > >
> > > > External Email
> > > >
> > > > 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, I didn't follow - what is all ABI?
> > > currently EFA implementation encodes the cachability inside the key,
> > > It's not exposed in ABI file and is opaque to user-space. The kernel
> > > decides on the cachability And get's it back in the key when mmap is
> > > called. It seems good enough for the current cases.
> > 
> > Then the key 'offset' should not include cachability information at all.
> > 
> Fair enough, so as you stated above the cachabiliy can be set in the cookie. 
> Would we still like to leave some bits for future ABI enhancements, requests, from user ? 
> Similar to a page type that mlx has ? 

Doesn't make sense to mix and match, the page_type was just some way
to avoid tracking cookies in some cases. If we are always having a
cookie then the cookie should indicate the type based on how it was
created. Totally opaque

Jason