Re: [RFC 7/11] virtio_pci: new, capability-aware driver.
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On Wed, Jan 11, 2012 at 8:46 PM, Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote: > 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 ? It's a corner case, the value is small. I also hit this with virtio on SPARC which is made difficult by the fact that the Solaris kernel assumes there is an IOMMU for scatter-gather and doesn't provide functions for allocating physically contiguous memory in drivers. It's another instance where this shortcut comes up against problems and behaving like a real PCI device would work fine. > Using an iommu is going to slaugher your performance, so at the very > least it should be kept an option. That's a good idea. By default it can continue to use physical addresses. I guess there's no point in worrying about it until we have a real user. Stefan _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization