Linux Virtualization - Apr 2016 - [PATCH RFC] fixup! virtio: convert to use DMA api

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 at
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

On Tue, Apr 19, 2016 at 3:27 AM, Michael S. Tsirkin <mst at 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 at
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

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 at
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 at
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