Re: [Qemu-devel] [RFC Design Doc v3] Enable Shared Virtual Memory feature in pass-through scenarios

[Konrad Rzeszutek Wilk]

On Wed, Nov 30, 2016 at 08:49:24AM +0000, Liu, Yi L wrote:
> What's changed from v2:
> a) Detailed feature description
> b) refine description in "Address translation in virtual SVM"
> b) "Terms" is added
> 
> Content
> ===============================================
> 1. Feature description
> 2. Why use it?
> 3. How to enable it
> 4. How to test
> 5. Terms
> 
> Details
> ===============================================
> 1. Feature description
> Shared virtual memory(SVM) is to let application program share its virtual
> address with SVM capable devices. 
> 
> Shared virtual memory details:
> a) SVM feature requires ATS/PRQ/PASID support on both device side and
> IOMMU side.
> b) SVM capable devices could send DMA requests with PASID, the address
> in the request would be a virtual address within a program's virtual address
> space.
> c) IOMMU would use first level page table to translate the address in the
> request.
> d) First level page table is a HVA->HPA mapping on bare metal.
> 
> Shared Virtual Memory feature in pass-through scenarios is actually SVM
> virtualization. It is to let application programs(running in guest)share their
> virtual address with assigned device(e.g. graphics processors or 
> accelerators).

I think I am missing something obvious, but the current way that DRM
works is that the kernel sets up its VA addresses for the GPU and it uses
that for its ring. It also setups an user level mapping for the GPU if the
application (Xorg) really wants it - but most of the time the kernel is
in charge of poking at the ring, and the memory that is shared with the
Xorg is normal RAM allocated via alloc_pages (see 
drivers/gpu/drm/ttm/ttm_page_alloc_dma.c
and drivers/gpu/drm/ttm/ttm_page_alloc.c).

So are talking about the guest applications having access to the 
ring of the GPU?