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

[Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>]

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>

Hi,

Thanks for pushing this forward. I got caught up in other stuff, perhaps
I'll be able to beat you to submitting the actual implementation soon. :-)

[snip]


	Eduard