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xen-devel
Re: [Xen-devel] Is there a way to map pv guest pseudo physical address i
On Mon, 27 Jun 2011, Tim Deegan wrote:
> At 17:26 +0800 on 27 Jun (1309195596), Wei Liu wrote:
> > > You need to arrange for the Xen specific bit of the virtio frontend to
> > > do the p2m translation. All externally visible addresses from a PV guest
> > > must be in MFN space.
> > >
> >
> > That's not a good approach. Dirvers reside on a higher level from the
> > "Xen specific bit". Doing this kind of translation will break their
> > generality. But there seems no other way to get this job done...
>
> This is a general problem with operating systems: bus addresses are not
> always the same as CPU memory addresses (even though in the easy case of
> a simple x86 PC they often are). This is why OSes already have
> interfaces to allocate DMA-able memory and to translate between the
> address that CPU will map and the one the peripheral should use. If
> your driver's not using the right address-space to program its hardware
> it is broken; the Xen PV model is just an extreme case of this.
Indeed.
When passing addresses to devices, drivers should use the DMA APIs to
make sure that proper address conversions take place (see
Documentation/DMA-API-HOWTO.txt).
On Xen the swiotlb implementation takes care of doing pfn to mfn
conversions as well.
For example, to allocate a consistent buffer that is addressable by both
a device and the cpu, the correct API is dma_alloc_coherent, that
returns two values: the virtual address which you can use to access it
from the CPU and dma_handle which you pass to the device.
I can very well imagine that virtio drivers are not doing any of this
because they didn't need to, but it would be probably OK to submit a
patch to convert the drivers to use the proper APIs.
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