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

Jason Gunthorpe
2018-11-22T02:58:45Z

On Wed, Nov 21, 2018 at 02:08:05PM +0800, Kenneth Lee wrote:

> > But considering Jean's SVA stuff seems based on mmu notifiers, I have
> > a hard time believing that it has any different behavior from RDMA's
> > ODP, and if it does have different behavior, then it is probably just
> > a bug in the ODP implementation.
>
> As Jean has explained, his solution is based on page table sharing. I think ODP
> should also consider this new feature.

Shared page tables would require the HW to walk the page table format
of the CPU directly, not sure how that would be possible for ODP?

Presumably the implementation for ARM relies on the IOMMU hardware
doing this?

> > > > If all your driver needs is to mmap some PCI bar space, route
> > > > interrupts and do DMA mapping then mediated VFIO is probably a good
> > > > choice.
> > >
> > > Yes. That is what is done in our RFCv1/v2. But we accepted Jerome's opinion and
> > > try not to add complexity to the mm subsystem.
> >
> > Why would a mediated VFIO driver touch the mm subsystem? Sounds like
> > you don't have a VFIO driver if it needs to do stuff like that...
>
> VFIO has no ODP-like solution, and if we want to solve the fork problem, we have
> to make some change to iommu and the fork procedure. Further, VFIO takes every
> queue as a independent device. This create a lot of trouble on resource
> management. For example, you will need a manager process to withdraw the unused
> device and you need to let the user process know about PASID of the queue, and
> so on.

Well, I would think you'd add SVA support to the VFIO driver as a
generic capability - it seems pretty useful for any VFIO user as it
avoids all the kernel upcalls to do memory pinning and DMA address
translation.

Once the VFIO driver knows about this as a generic capability then the
device it exposes to userspace would use CPU addresses instead of DMA
addresses.

The question is if your driver needs much more than the device
agnostic generic services VFIO provides.

I'm not sure what you have in mind with resource management.. It is
hard to revoke resources from userspace, unless you are doing
kernel syscalls, but then why do all this?

Jason