Michael S. Tsirkin wrote: > On Wed, Sep 16, 2009 at 10:10:55AM -0400, Gregory Haskins wrote: >>> There is no role reversal. >> So if I have virtio-blk driver running on the x86 and vhost-blk device >> running on the ppc board, I can use the ppc board as a block-device. >> What if I really wanted to go the other way? > > It seems ppc is the only one that can initiate DMA to an arbitrary > address, so you can't do this really, or you can by tunneling each > request back to ppc, or doing an extra data copy, but it's unlikely to > work well. > > The limitation comes from hardware, not from the API we use. Understood, but presumably it can be exposed as a sub-function of the ppc's board's register file as a DMA-controller service to the x86. This would fall into the "tunnel requests back" category you mention above, though I think "tunnel" implies a heavier protocol than it would actually require. This would look more like a PIO cycle to a DMA controller than some higher layer protocol. You would then utilize that DMA service inside the memctx, and it the rest of vbus would work transparently with the existing devices/drivers. I do agree it would require some benchmarking to determine its feasibility, which is why I was careful to say things like "may work" ;). I also do not even know if its possible to expose the service this way on his system. If this design is not possible or performs poorly, I admit vbus is just as hosed as vhost in regard to the "role correction" benefit. Kind Regards, -Greg
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