linux-kernel - Re: [PATCH v3 0/3] virtio DMA API core stuff

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Date:	Mon, 23 Nov 2015 00:11:09 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	marcel@...hat.com, Joerg Roedel <jroedel@...e.de>,
	linux-s390 <linux-s390@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Andy Lutomirski <luto@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Christoph Hellwig <hch@....de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff

On Sun, Nov 22, 2015 at 03:54:21PM +0000, David Woodhouse wrote:
> On Sun, 2015-11-22 at 15:06 +0200, Marcel Apfelbaum wrote:
> > 
> > 
> > I tried to generate a DMAR table that excludes some devices from
> > IOMMU translation, however it does not help.
> > 
> > The reason is, as far as I understand, that Linux kernel does
> > not allow any device being outside an IOMMU scope if the
> > iommu kernel option is activated.
> > 
> > Does anybody know if it is "by design" or is simply an uncommon
> > configuration?
> > (some devices in an IOMMU scope, while others outside *any* IOMMU
> > scope)
> 
> That's a kernel bug in the way it handles per-device DMA operations. Or
> more to the point, in the way it doesn't — the non-translated devices
> end up being pointed to the intel_dma_ops despite the fact they
> shouldn't be. I'm working on that...
> 
> -- 
> dwmw2
> 

Interesting. This seems to imply such configurations aren't
common, so I wonder whether other guest OS-es treat them
correctly.

If many of them are, we probably shouldn't use this in QEMU:
we care about guests actually working :)

-- 
MST
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