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

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

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@redhat.com> 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@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.
> >
> 
> 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