On Wed, 2012-01-11 at 14:28 +0000, Stefan Hajnoczi wrote: > On Wed, Jan 11, 2012 at 9:10 AM, Benjamin Herrenschmidt > <benh@xxxxxxxxxxxxxxxxxxx> wrote: > > On Wed, 2012-01-11 at 08:47 +0000, Stefan Hajnoczi wrote: > >> > >> This is also an opportunity to stop using CPU physical addresses in > >> the ring and instead perform DMA like a normal PCI device (use bus > >> addresses). > > > > Euh why ? > > Because it's a paravirt hack that ends up hitting corner cases. It's > not possible to do virtio-pci passthrough under nested virtualization > unless we use an IOMMU. Imagine passing virtio-net from L0 into the > L2 guest (i.e. PCI-passthrough). If virtio-pci is really "PCI" this > should be possible but it's not when we use physical addresses instead > of bus addresses. Is this just an academic exercise or is there any actual value in doing this ? Using an iommu is going to slaugher your performance, so at the very least it should be kept an option. Yes, it's a paravirt "hack" as you call it but that's what virtio is all about.... paravirt. If you prefer you can emulate a real HW device :-) Ben. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
