Re: [PATCH] VFIO driver: Non-privileged user level PCI drivers

[Avi Kivity]

On 06/02/2010 08:29 AM, Chris Wright wrote:
> * Avi Kivity (avi@xxxxxxxxxx) wrote:
>    
> > On 06/02/2010 12:26 AM, Tom Lyon wrote:
> >      
> > > I'm not really opposed to multiple devices per domain, but let me point out how I
> > > ended up here.  First, the driver has two ways of mapping pages, one based on the
> > > iommu api and one based on the dma_map_sg api.  With the latter, the system
> > > already allocates a domain per device and there's no way to control it. This was
> > > presumably done to help isolation between drivers.  If there are multiple drivers
> > > in the user level, do we not want the same isoation to apply to them?
> > >        
> > In the case of kvm, we don't want isolation between devices, because
> > that doesn't happen on real hardware.
> >      
> Sure it does.  That's exactly what happens when there's an iommu
> involved with bare metal.
>    

But we are emulating a machine without an iommu.

When we emulate a machine with an iommu, then yes, we'll want to use as 
many domains as the guest does.

> > So if the guest programs
> > devices to dma to each other, we want that to succeed.
> >      
> And it will as long as ATS is enabled (this is a basic requirement
> for PCIe peer-to-peer traffic to succeed with an iommu involved on
> bare metal).
>
> That's how things currently are, i.e. we put all devices belonging to a
> single guest in the same domain.  However, it can be useful to put each
> device belonging to a guest in a unique domain.  Especially as qemu
> grows support for iommu emulation, and guest OSes begin to understand
> how to use a hw iommu.
>    

Right, we need to keep flexibility.

> > > And then there's the fact that it is possible to have multiple disjoint iommus on a system,
> > > so it may not even be possible to bring 2 devices under one domain.
> > >        
> > That's indeed a deficiency.
> >      
> Not sure it's a deficiency.  Typically to share page table mappings
> across multiple iommu's you just have to do update/invalidate to each
> hw iommu that is sharing the mapping.  Alternatively, you can use more
> memory and build/maintain identical mappings (as Tom alludes to below).
>    

Sharing the page tables is just an optimization, I was worried about 
devices in separate domains not talking to each other.  if ATS fixes 
that, great.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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