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

[Andy Lutomirski]

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