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

[Tian, Kevin]

> From: Neo Jia [mailto:address@hidden
> 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);

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