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

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Date:	Wed, 27 Apr 2016 17:34:30 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	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, peterx@...hat.com,
	linux-kernel@...r.kernel.org, Amit Shah <amit.shah@...hat.com>,
	iommu@...ts.linux-foundation.org,
	Stefan Hajnoczi <stefanha@...hat.com>, kvm@...r.kernel.org,
	cornelia.huck@...ibm.com, pbonzini@...hat.com,
	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 04:23:32PM +0200, Joerg Roedel 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.

QEMU can choose to bypass IOMMU for one device and not the other.
IOMMU in QEMU isn't involved when it's bypassed.

> So any fix here belongs
> into the platform/iommu code too and not into some driver.

Fine but this is beside the point. Almost all virtio devices
bypass IOMMU and what this patch does is create a way
to detect devices that don't. This code can maybe go into
platform.

> > 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.
> 
> 
> 
> 	Joerg

It would be easy if they did but they don't do this on all systems.
In particular the idea for firmware interface was clearly
that a given bus either is connected through IOMMU or bypassing it.
Whether virtio bypasses the iommu is unrelated to the bus it's on.

-- 
MST