linux-kernel - Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

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Date:	Wed, 27 Apr 2016 17:54:57 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Joerg Roedel <joro@...tes.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Kevin Wolf <kwolf@...hat.com>, Wei Liu <wei.liu2@...rix.com>,
	Andy Lutomirski <luto@...nel.org>, qemu-block@...gnu.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Jason Wang <jasowang@...hat.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	"qemu-devel@...gnu.org Developers" <qemu-devel@...gnu.org>,
	peterx@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Amit Shah <amit.shah@...hat.com>,
	iommu@...ts.linux-foundation.org,
	Stefan Hajnoczi <stefanha@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Anthony PERARD <anthony.perard@...rix.com>
Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

On Wed, Apr 27, 2016 at 07:43:07AM -0700, Andy Lutomirski wrote:
> On Wed, Apr 27, 2016 at 7:38 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > On Wed, Apr 27, 2016 at 07:31:43AM -0700, Andy Lutomirski wrote:
> >> On Wed, Apr 27, 2016 at 7:23 AM, Joerg Roedel <joro@...tes.org> wrote:
> >> > On Wed, Apr 27, 2016 at 04:37:04PM +0300, Michael S. Tsirkin wrote:
> >> >> One correction: it's a feature of the device in the system.
> >> >> There could be a mix of devices bypassing and not
> >> >> bypassing the IOMMU.
> >> >
> >> > No, it really is not. A device can't chose to bypass the IOMMU. But the
> >> > IOMMU can chose to let the device bypass. So any fix here belongs
> >> > into the platform/iommu code too and not into some driver.
> >> >
> >> >> Sounds good. And a way to detect appropriate devices could
> >> >> be by looking at the feature flag, perhaps?
> >> >
> >> > Again, no! The way to detect that is to look into the iommu description
> >> > structures provided by the firmware. They provide everything necessary
> >> > to tell the iommu code which devices are not translated.
> >> >
> >>
> >> Except on PPC and SPARC.  As far as I know, those are the only
> >> problematic platforms.
> >>
> >> Is it too late to *disable* QEMU's q35-iommu thingy until it can be
> >> fixed to report correct data in the DMAR tables?
> >>
> >> --Andy
> >
> > Meaning virtio or assigned devices?
> > For virtio - it's way too late since these are working configurations.
> > For assigned devices - they don't work on x86 so it doesn't have
> > to be disabled, it's safe to ignore.
> 
> I mean actually prevent QEMU from running in q35-iommu mode with any
> virtio devices attached or maybe even turn off q35-iommu mode entirely
> [1].  Doesn't it require that the user literally pass the word
> "experimental" into QEMU right now?  It did at some point IIRC.
> 
> The reason I'm asking is that, other than q35-iommu, QEMU's virtio
> devices *don't* bypass the IOMMU except on PPC and SPARC, simply
> because there is no other configuration AFAICT that has virtio and and
> IOMMU.  So maybe the right solution is to fix q35-iommu to use DMAR
> correctly (thus breaking q35-iommu users with older guest kernels,
> which hopefully don't actually exist) and to come up with a PPC- and
> SPARC-specific solution, or maybe OpenFirmware-specific solution, to
> handle PPC and SPARC down the road.
> 
> [1] I'm pretty sure I emailed the QEMU list before q35-iommu ever
> showed up in a release asking the QEMU team to please not do that
> until this issue was resolved.  Sadly, that email was ignored :(
> 
> --Andy

Sorry, I didn't make myself clear.
Point is, QEMU is not the only virtio implementation out there.
So we can't know no virtio implementations have an IOMMU as long as
linux supports this IOMMU.
virtio always used physical addresses since it was born and if it
changes that it must do this in a way that does not break existing
users.

-- 
MST