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

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Date:	Tue, 19 Apr 2016 19:09:03 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	David Woodhouse <dwmw2@...radead.org>,
	"qemu-devel@...gnu.org Developers" <qemu-devel@...gnu.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, peterx@...hat.com,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Kevin Wolf <kwolf@...hat.com>,
	Amit Shah <amit.shah@...hat.com>, qemu-block@...gnu.org,
	Jason Wang <jasowang@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH RFC] fixup! virtio: convert to use DMA api

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@...hat.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@...radead.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