Re: [Qemu-devel] [PATCH RFC] fixup! virtio: convert to use DMA api

["Michael S. Tsirkin" <mst@redhat.com>]

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@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