Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
> Currently, a lot of the virtio code assumes that bus (i.e. hypervisor)
> addresses are the same as physical address. This is false on Xen, so
> virtio is completely broken. I wouldn't be surprised if it also
> becomes a problem the first time that someone sticks a physical
> "virtio" device on a 32-bit bus on an ARM SOC with more than 4G RAM.
>
> Would you accept patches to convert virtio_ring and virtio_pci to use
> the DMA APIs? I think that the only real catch will be that
> virtio_ring's approach to freeing indirect blocks is currently
> incompatible with the DMA API -- it assumes that knowing the bus
> address is enough to call kfree, and I don't think that the DMA API
> provides a reverse mapping like that.
Hi Andy,
This has long been a source of contention. virtio assumes that
the hypervisor can decode guest-physical addresses.
PowerPC, in particular, doesn't want to pay the cost of IOMMU
manipulations, and all arguments presented so far for using an IOMMU for
a virtio device are weak. And changing to use DMA APIs would break them
anyway.
Of course, it's Just A Matter of Code, so it's possible to
create a Xen-specific variant which uses the DMA APIs. I'm not sure
what that would look like in the virtio standard, however.
Cheers,
Rusty.
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