Re: [PATCH v5 2/3] virtio_pci: Use the DMA API for virtqueues when possible

On Sat, Sep 20, 2014 at 10:05 PM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Sun, 2014-09-21 at 15:03 +1000, Benjamin Herrenschmidt wrote:

quoted

The exception I mentioned is that I would really like the virtio device
to expose via whatever transport we chose to use (though capability
exchange sounds like a reasonable one) whether the "server"
implementation is bypassing IOMMUs or not instead on relying on client
side heuristics.

IE. Basically, we are trying to "guess" with an ifdef CONFIG_PPC, what
is essentially an attribute of the server-side, ie, whether is bypasses
the iommu for the PCI bus it resides on.

quoted

I believe all the arguments about whether this should be a bus property
or whether the x86 case can be worked around via ACPI tables etc... are
all moot. Today, qemu implementation can put virtio devices on busses
with an iommu and bypass it, so at the very least for backward
compatibility, we should expose that appropriately from the "server"
side.

And of course, since we are talking about backward compatibility with
existing qemus here, the capability should be the opposite, ie "honor
iommu", with the assumption that without it, the implementation bypasses
it, which reflects what the current qemu implementation does on any
architecture, whether you configure the bus to have an iommu emulated on
it or not.

Can PPC do this using a new devicetree property?

--Andy