On Mon, Apr 18, 2016 at 12:24:15PM -0700, Andy Lutomirski wrote: > On Mon, Apr 18, 2016 at 11:29 AM, David Woodhouse <dw...@infradead.org> 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. -- MST
