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

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Date:	Wed, 27 Apr 2016 17:38:51 +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 07:31:43AM -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.

-- 
MST