Re: [RFC PATCH v2 3/3] VFIO: Type1 IOMMU mapping support for vGPU

[Neo Jia <cjia@nvidia.com>]

On Fri, Mar 11, 2016 at 04:46:23AM +0000, Tian, Kevin wrote:
> > From: Neo Jia [mailto:cjia@nvidia.com]
> > Sent: Friday, March 11, 2016 12:20 PM
> > 
> > On Thu, Mar 10, 2016 at 11:10:10AM +0800, Jike Song wrote:
> > >
> > > >> Is it supposed to be the caller who should set
> > > >> up IOMMU by DMA api such as dma_map_page(), after calling
> > > >> vgpu_dma_do_translate()?
> > > >>
> > > >
> > > > Don't think you need to call dma_map_page here. Once you have the pfn available
> > > > to your GPU kernel driver, you can just go ahead to setup the mapping as you
> > > > normally do such as calling pci_map_sg and its friends.
> > > >
> > >
> > > Technically it's definitely OK to call DMA API from the caller rather than here,
> > > however personally I think it is a bit counter-intuitive: IOMMU page tables
> > > should be constructed within the VFIO IOMMU driver.
> > >
> > 
> > Hi Jike,
> > 
> > For vGPU, what we have is just a virtual device and a fake IOMMU group, therefore
> > the actual interaction with the real GPU should be managed by the GPU vendor driver.
> > 
> 
> Hi, Neo,
> 
> Seems we have a different thought on this. Regardless of whether it's a virtual/physical 
> device, imo, VFIO should manage IOMMU configuration. The only difference is:
> 
> - for physical device, VFIO directly invokes IOMMU API to set IOMMU entry (GPA->HPA);
> - for virtual device, VFIO invokes kernel DMA APIs which indirectly lead to IOMMU entry 
> set if CONFIG_IOMMU is enabled in kernel (GPA->IOVA);

How does it make any sense for us to do a dma_map_page for a physical device that we don't 
have any direct interaction with?

> 
> This would provide an unified way to manage the translation in VFIO, and then vendor
> specific driver only needs to query and use returned IOVA corresponding to a GPA. 
> 
> Doing so has another benefit, to make underlying vGPU driver VMM agnostic. For KVM,
> yes we can use pci_map_sg. However for Xen it's different (today Dom0 doesn't see
> IOMMU. In the future there'll be a PVIOMMU implementation) so different code path is 
> required. It's better to abstract such specific knowledge out of vGPU driver, which just
> uses whatever dma_addr returned by other agent (VFIO here, or another Xen specific
> agent) in a centralized way.
> 
> Alex, what's your opinion on this?
> 
> Thanks
> Kevin