[vfio-users] IOMMU page table of QEMU vfio device passthrough

[Huaicheng Li]

Hi Alex,

This is why we have an IOMMU.  The guest drivers and the device operate
> in the guest physical address space, the IOMMU handles the translation
> to the host physical address space.


Thanks. I guess this is the answer I need. I will check the code to have a
better understanding.

Again, thank you!

Best,
Huaicheng

On Tue, Jun 5, 2018 at 9:59 PM Alex Williamson <alex.williamson at redhat.com>
wrote:

> On Tue, 5 Jun 2018 18:52:43 -0700
> Huaicheng Li <huaicheng at cs.uchicago.edu> wrote:
>
> > Hi Alex,
> >
> > VFIO does not setup
> > > mappings per DMA buffer, it maps all of guest physical memory through
> > > the IOMMU so that the device can DMA to any GPA and it's already
> > > mapped.  We call this transparent assignment, the guest can perform DMA
> > > without any knowledge of or interaction with the host IOMMU.
> >
> >
> > Could you briefly explain how all of guest physical memory is mapped
> > through IOMMU.
> > I do see a ``vfio_dma_map`` call with `size` almost equal to my guest
> > memory size, is
> > this the place where IOMMU is instructed to enable such transparent
> > assignment you
> > mentioned?
>
> I don't know that I have a brief explanation for this, you'll need to
> look at the code.  The entry point to vfio is the MemoryListener (which
> calls vfio_dma_map) attached to the device AddressSpace, which is the
> system memory address space when a guest IOMMU is not present.
> RAMBlocks are mapped through MemoryRegions and sections of those come
> in through the MemoryListner to be populated through the IOMMU.
>
> > Since the guest driver is issuing IOs with GPA, how does the
> > device do
> > DMA to the corresponding physical DMA address?
>
> This is why we have an IOMMU.  The guest drivers and the device operate
> in the guest physical address space, the IOMMU handles the translation
> to the host physical address space.
>
> > Is the GPA->HPA a static
> > mapping
> > so the HPA can be calculated instead of walking the page tables (which
> > those DMA APIs
> > do)?
>
> Without a guest IOMMU, the translations are almost entirely static, for
> instance the RAM in your computer doesn't spontaneously decide to
> relocate itself in the physical address space of your system, neither
> does RAM in a VM.  All of guest physical memory is pinned in the host
> and mapped through the IOMMU.  Regardless of this, there is no page
> table walking to create these mappings.  Each RAMBlock is just a range
> of memory with a start and end offset in the guest physical address
> space.  Thanks,
>
> Alex
>
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