On Wed, Aug 27, 2014 at 01:52:50PM +0200, Michael S. Tsirkin wrote: > On Wed, Aug 27, 2014 at 08:40:51PM +0930, Rusty Russell wrote: > > 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. > > For x86 as of QEMU 2.0 there's no iommu. > So a reasonable thing to do for that platform > might be to always use iommu *if it's there*. > My understanding is this isn't the case for powerpc? Wasn't there some implementation of AMD IOMMU code on QEMU mailing list floating around? Aha: https://www.mail-archive.com/kvm@xxxxxxxxxxxxxxx/msg40516.html _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
