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

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Date:	Mon, 18 Apr 2016 07:58:37 -0400
From:	David Woodhouse <dwmw2@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>, qemu-devel@...gnu.org,
	linux-kernel@...r.kernel.org
Cc:	pbonzini@...hat.com, peterx@...hat.com, 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>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [PATCH RFC] fixup! virtio: convert to use DMA api

On Mon, 2016-04-18 at 14:47 +0300, Michael S. Tsirkin wrote:
> This adds a flag to enable/disable bypassing the IOMMU by
> virtio devices.

I'm still deeply unhappy with having this kind of hack in the virtio
code at all, as you know. Drivers should just use the DMA API and if
the *platform* wants to make it a no-op for a specific device, then it
can.

Remember, this isn't just virtio either. Don't we have *precisely* the
same issue with assigned PCI devices on a system with an emulated Intel
IOMMU? The assigned PCI devices aren't covered by the emulated IOMMU,
and the platform needs to know to bypass *those* too.

Now, we've had this conversation, and we accepted the hack in virtio
for now until the platforms (especially SPARC and Power IIRC) can get
their act together and make their DMA API implementations not broken.

But now you're adding this hack to the public API where we have to
support it for ever. Please, can't we avoid that?

-- 
dwmw2

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