On Tue, Apr 19, 2016 at 09:02:14AM -0700, Andy Lutomirski wrote: > On Tue, Apr 19, 2016 at 3:27 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Mon, Apr 18, 2016 at 12:24:15PM -0700, Andy Lutomirski wrote: > >> On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote: > >> > For x86, you *can* enable virtio-behind-IOMMU if your DMAR tables tell > >> > the truth, and even legacy kernels ought to cope with that. > >> > FSVO 'ought to' where I suspect some of them will actually crash with a > >> > NULL pointer dereference if there's no "catch-all" DMAR unit in the > >> > tables, which puts it back into the same camp as ARM and Power. > >> > >> I think x86 may get a bit of a free pass here. AFAIK the QEMU IOMMU > >> implementation on x86 has always been "experimental", so it just might > >> be okay to change it in a way that causes some older kernels to OOPS. > >> > >> --Andy > > > > Since it's experimental, it might be OK to change *guest kernels* > > such that they oops on old QEMU. > > But guest kernels were not experimental - so we need a QEMU mode that > > makes them work fine. The more functionality is available in this QEMU > > mode, the betterm because it's going to be the default for a while. For > > the same reason, it is preferable to also have new kernels not crash in > > this mode. > > > > People add QEMU features that need new guest kernels all time time. > If you enable virtio-scsi and try to boot a guest that's too old, it > won't work. So I don't see anything fundamentally wrong with saying > that the non-experimental QEMU Q35 IOMMU mode won't boot if the guest > kernel is too old. It might be annoying, since old kernels do work on > actual Q35 hardware, but it at least seems to be that it might be > okay. > > --Andy Yes but we need a mode that makes both old and new kernels work, and that should be the default for a while. this is what the IOMMU_PASSTHROUGH flag was about: old kernels ignore it and bypass DMA API, new kernels go "oh compatibility mode" and bypass the IOMMU within DMA API. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
