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Re: [Xen-devel] Essay on an important Xen decision (long)

Le Mardi 10 Janvier 2006 20:55, Anthony Liguori a écrit :
> Hi Dan,
> Thanks for the thorough explaination of physical memory virtualization.
> It's a topic that there isn't a lot of good reference on.
> You seem to conclude that the only possible solutions are making the
> dom0 either P==M or P2M.  Is it not possible to make dom0 VP?
> If the only issue for making dom0 VP is DMA, wouldn't it be easier to
> modify the Linux DMA subsystem[1] to make a special hypercall to
> essentially pin a VP to a particular MFN that could be used for the
> DMA?  One could imagine the hypervisor reversing low memory specifically
> for DMA such that bounce buffers could be avoided too.
> VP makes a lot of interesting memory optimizations considerably easier
> (memory compacting, swapping, etc.).
> [1] Realizing that I know very little about the Linux DMA subsystem so I
> don't know if this is outside the realm of possibilities.

a few years ago (it was with linux 2.2), I wrote device drivers for rather 
complex hardware.  DMA subsystem didn't really exist.  The main reason is an 
hardware reason: DMA chip do not exist anymore because nowaday (almost since 
PCI) every driver chip do DMA by itself.


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