Date: Tue, 19 Apr 2016 13:27:15 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: David Woodhouse <dwmw2@infradead.org>,
        "qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, peterx@redhat.com,
        Cornelia Huck <cornelia.huck@de.ibm.com>,
        Stefan Hajnoczi <stefanha@redhat.com>, Kevin Wolf <kwolf@redhat.com>,
        Amit Shah <amit.shah@redhat.com>, qemu-block@nongnu.org,
        Jason Wang <jasowang@redhat.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Wei Liu <wei.liu2@citrix.com>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        kvm list <kvm@vger.kernel.org>
Subject: Re: [PATCH RFC] fixup! virtio: convert to use DMA api
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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