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SubjectRe: [Alacrityvm-devel] [PATCH v3 3/6] vbus: add a "vbus-proxy" bus model for vbus_driver objects
On 08/19/2009 12:26 AM, Avi Kivity wrote:
>> Off the top of my head, I would think that transporting userspace
>> addresses in the ring (for copy_(to|from)_user()) vs. physical addresses
>> (for DMAEngine) might be a problem. Pinning userspace pages into memory
>> for DMA is a bit of a pain, though it is possible.
> Oh, the ring doesn't transport userspace addresses. It transports
> guest addresses, and it's up to vhost to do something with them.
> Currently vhost supports two translation modes:
> 1. virtio address == host virtual address (using copy_to_user)
> 2. virtio address == offsetted host virtual address (using copy_to_user)
> The latter mode is used for kvm guests (with multiple offsets,
> skipping some details).
> I think you need to add a third mode, virtio address == host physical
> address (using dma engine). Once you do that, and wire up the
> signalling, things should work.

You don't need in fact a third mode. You can mmap the x86 address space
into your ppc userspace and use the second mode. All you need then is
the dma engine glue and byte swapping.

I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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