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

From: "Michael S. Tsirkin" <mst@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: linux-s390 <linux-s390@vger.kernel.org>,
	Joerg Roedel <jroedel@suse.de>, KVM <kvm@vger.kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Sebastian Ott <sebott@linux.vnet.ibm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Andy Lutomirski <luto@kernel.org>,
	marcel@redhat.com, Paolo Bonzini <pbonzini@redhat.com>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff
Date: Mon, 23 Nov 2015 00:11:09 +0200	[thread overview]
Message-ID: <20151123000329-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <1448207661.89124.50.camel@infradead.org>

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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