On Thu, Aug 28, 2014 at 07:31:16AM +1000, Benjamin Herrenschmidt wrote: > On Wed, 2014-08-27 at 20:40 +0930, Rusty Russell wrote: > > > 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. > > So this has popped up in the past a few times already from people who > want to use virtio as a transport between physical systems connected > via a bus like PCI using non-transparent bridges for example. > > There's a way to get both here that isn't too nasty... we can make the > virtio drivers use the dma_map_* APIs and just switch the dma_ops in > the struct device based on the hypervisor requirements. IE. For KVM we > could attach a set of ops that basically just return the physical > address, real PCI transport would use the normal callbacks etc...
Right. > > The only problem at the moment is that the dma_map_ops, while > defined generically, aren't plumbed into the generic struct device > but instead on some architectures dev_archdata. This includes > powerpc, ARM and x86 (under a CONFIG option for the latter which > is only enabled on x86_64 and some oddball i386 variant). I am not following the interaction between 'struct device', 'struct dev_archdata' and 'struct dma_map_ops' ? The 'struct dma_ops' should be able to exist without having to exist in the other structures? Naturally the implementation of 'struct dma_ops' has to use 'struct device' otherwise it can't get the details such as dma_mapping. > > So either we switch to have all architectures we care about always > use the generic DMA ops and move the pointer to struct device, or > we create another inline "indirection" to deal with the cases > without the dma_map_ops... Or you implement an passthrough 'dma_map_ops' that you suggested? Thought I feel I am not groking something from your email. Hmm, time to get some more coffee. > > Cheers, > Ben. > > _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
