Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

[Barry Song]

On Fri, Oct 8, 2021 at 12:32 AM Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
>
> On Thu, Oct 07, 2021 at 06:43:33PM +1300, Barry Song wrote:
>
> > So do we have a case where devices can directly access the kernel's data
> > structure such as a list/graph/tree with pointers to a kernel virtual address?
> > then devices don't need to translate the address of pointers in a structure.
> > I assume this is one of the most useful features userspace SVA can provide.
>
> AFIACT that is the only good case for KVA, but it is also completely
> against the endianess, word size and DMA portability design of the
> kernel.
>
> Going there requires some new set of portable APIs for gobally
> coherent KVA dma.

yep. I agree. it would be very weird if accelerators/gpu are sharing
kernel' data struct, but for each "DMA" operation - reading or writing
the data struct, we have to call dma_map_single/sg or
dma_sync_single_for_cpu/device etc. It seems once devices and cpus
are sharing virtual address(SVA), code doesn't need to do explicit
map/sync each time.

>
> Jason

Thanks
barry