Re: [Qemu-devel] [PATCH 4/8] Add universal DMA helper functions

[David Gibson <david@gibson.dropbear.id.au>]

On Sun, Feb 26, 2012 at 12:04:49PM +0200, Michael S. Tsirkin wrote:
> On Fri, Feb 24, 2012 at 02:27:39PM +1100, David Gibson wrote:
> > Not that long ago, every device implementation using DMA directly
> > accessed guest memory using cpu_physical_memory_*().  This meant that
> > adding support for a guest visible IOMMU would require changing every
> > one of these devices to go through IOMMU translation.
> > 
> > Shortly before qemu 1.0, I made a start on fixing this by providing
> > helper functions for PCI DMA.  These are currently just stubs which
> > call the direct access functions, but mean that an IOMMU can be
> > implemented in one place, rather than for every PCI device.
> > 
> > Clearly, this doesn't help for non PCI devices, which could also be
> > IOMMU translated on some platforms.  It is also problematic for the
> > devices which have both PCI and non-PCI version (e.g. OHCI, AHCI) - we
> > cannot use the the pci_dma_*() functions, because they assume the
> > presence of a PCIDevice, but we don't want to have to check between
> > pci_dma_*() and cpu_physical_memory_*() every time we do a DMA in the
> > device code.
> > 
> > This patch makes the first step on addressing both these problems, by
> > introducing new (stub) dma helper functions which can be used for any
> > DMA capable device.
> > 
> > These dma functions take a DMAContext *, a new (currently empty)
> > variable describing the DMA address space in which the operation is to
> > take place.  NULL indicates untranslated DMA directly into guest
> > physical address space.  The intention is that in future non-NULL
> > values will given information about any necessary IOMMU translation.
> > 
> > DMA using devices must obtain a DMAContext (or, potentially, contexts)
> > from their bus or platform.  For now this patch just converts the PCI
> > wrappers to be implemented in terms of the universal wrappers,
> > converting other drivers can take place over time.
> > 
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Joerg Rodel <Joerg.Rodel@amd.com>
> > Cc: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
> > Cc: Richard Henderson <rth@twiddle.net>
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> 
> I'm a bit confused with all the stubbing going on.
> Is this the final form of the pci_* functions or just
> a stub? If the final form, we probably should just
> open-code them - they don't buy us much.
> If not, let's add a comment?

Well.. it's the intended final form of pci_dma_*() - which do become
trivial wrappers, yes.  It's _not_ the intended final form of dma_*(),
which need to grow code to do actual IOMMU translation.  I'll add a
comment about this in the next round.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
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