On Sep 3, 2014 5:11 AM, "Paolo Bonzini" <pbonzini@xxxxxxxxxx> wrote: > > Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto: > > On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote: > >> > >> IOMMU support for x86 is going to go in this week. > > > > But won't that break virtio on x86 ? Or will virtio continue bypassing > > it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call > > the dma mappings ops. > > > >> However, it is and likely will remain niche enough that I don't really > >> care about performance loss from IOMMU support. If you enable it, you > >> want it. > >> > >> So from the QEMU point of view we can simply add the direct-ram-access > >> property, and have the pseries machine turn it on by default (while > >> other machines can leave it off by default---they have no IOMMU and > >> thus no performance cost). > > > > Well, it's only for virtio and should be on by default on x86 as well if > > an iommu is installed no ? > > Yes, only for virtio---but for x86 I think it should be off by default, > even if that means virtio+IOMMU requires a new kernel. Just to clarify: is "it" the direct-ram-access property? If so, I think I might agree. Alternatively, could QEMU easily teach the IOMMU code to generate the ACPI tables such that virtio-pci devices aren't advertised as living behind the IOMMU? This would work both with and without my patches. On the other hand, maybe this gets complicated when hotplug is involved. --Andy > > Paolo _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
