Re: [RFCv3 PATCH 1/6] uacce: Add documents for WarpDrive/uacce

[Jerome Glisse]

On Mon, Nov 19, 2018 at 07:19:04PM +0000, Christopher Lameter wrote:
> On Mon, 19 Nov 2018, Jerome Glisse wrote:
> 
> > > IIRC this is solved in IB by automatically calling
> > > madvise(MADV_DONTFORK) before creating the MR.
> > >
> > > MADV_DONTFORK
> > >   .. This is useful to prevent copy-on-write semantics from changing the
> > >   physical location of a page if the parent writes to it after a
> > >   fork(2) ..
> >
> > This would work around the issue but this is not transparent ie
> > range marked with DONTFORK no longer behave as expected from the
> > application point of view.
> 
> Why would anyone expect a range registered via MR behave as normal? Device
> I/O is going on into that range. Memory is already special.
> 
> 
> > Also it relies on userspace doing the right thing (which is not
> > something i usualy trust :)).
> 
> This has been established practice for 15 or so years in a couple of use
> cases. Again user space already has to be doing special things in order to
> handle RDMA is that area.

Yes RDMA as an existing historical track record and thus people
should now be aware of its limitation. What i am fighting against
is new addition to kernel that pretend to do SVA (share virtual
address) while their hardware is not realy doing SVA. SVA with
IOMMU and ATS/PASID is fine, SVA in software with device driver
that abide to mmu notifier is fine. Anything else is not.

So i am just worrying about new user and making sure they under-
stand what is happening and not sell to their user something
false.

Cheers,
Jérôme