Re: [Qemu-devel] [RFC PATCH v3 3/3] VFIO Type1 IOMMU change: to support with iommu and without iommu

[Neo Jia]

On Fri, May 13, 2016 at 02:22:37PM +0800, Jike Song wrote:
> On 05/13/2016 10:41 AM, Tian, Kevin wrote:
> >> From: Neo Jia [mailto:address@hidden
> >> Sent: Friday, May 13, 2016 3:49 AM
> >>
> >>>
> >>>> Perhaps one possibility would be to allow the vgpu driver to register
> >>>> map and unmap callbacks.  The unmap callback might provide the
> >>>> invalidation interface that we're so far missing.  The combination of
> >>>> map and unmap callbacks might simplify the Intel approach of pinning the
> >>>> entire VM memory space, ie. for each map callback do a translation
> >>>> (pin) and dma_map_page, for each unmap do a dma_unmap_page and release
> >>>> the translation.
> >>>
> >>> Yes adding map/unmap ops in pGPU drvier (I assume you are refering to
> >>> gpu_device_ops as
> >>> implemented in Kirti's patch) sounds a good idea, satisfying both: 1)
> >>> keeping vGPU purely
> >>> virtual; 2) dealing with the Linux DMA API to achive hardware IOMMU
> >>> compatibility.
> >>>
> >>> PS, this has very little to do with pinning wholly or partially. Intel 
> >>> KVMGT has
> >>> once been had the whole guest memory pinned, only because we used a 
> >>> spinlock,
> >>> which can't sleep at runtime.  We have removed that spinlock in our 
> >>> another
> >>> upstreaming effort, not here but for i915 driver, so probably no biggie.
> >>>
> >>
> >> OK, then you guys don't need to pin everything. The next question will be 
> >> if you
> >> can send the pinning request from your mediated driver backend to request 
> >> memory
> >> pinning like we have demonstrated in the v3 patch, function vfio_pin_pages 
> >> and
> >> vfio_unpin_pages?
> >>
> > 
> > Jike can you confirm this statement? My feeling is that we don't have such 
> > logic
> > in our device model to figure out which pages need to be pinned on demand. 
> > So
> > currently pin-everything is same requirement in both KVM and Xen side...
> 
> [Correct me in case of any neglect:)]
> 
> IMO the ultimate reason to pin a page, is for DMA. Accessing RAM from a GPU is
> certainly a DMA operation. The DMA facility of most platforms, IGD and NVIDIA
> GPU included, is not capable of faulting-handling-retrying.
> 
> As for vGPU solutions like Nvidia and Intel provide, the memory address region
> used by Guest for GPU access, whenever Guest sets the mappings, it is
> intercepted by Host, so it's safe to only pin the page before it get used by
> Guest. This probably doesn't need device model to change :)

Hi Jike

Just out of curiosity, how does the host intercept this before it goes on the
bus?

Thanks,
Neo

> 
> 
> --
> Thanks,
> Jike
>