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

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David Woodhouse <dwmw2@infradead.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Sebastian Ott <sebott@linux.vnet.ibm.com>,
	linux-s390 <linux-s390@vger.kernel.org>,
	Cornelia Huck <cornelia.huck@de.ibm.com>,
	Joerg Roedel <jroedel@suse.de>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>, KVM <kvm@vger.kernel.org>
Subject: Re: [PATCH v3 0/3] virtio DMA API core stuff
Date: Fri, 20 Nov 2015 10:34:18 +0200	[thread overview]
Message-ID: <20151120102141-mutt-send-email-mst@redhat.com> (raw)
In-Reply-To: <1447988199.3016.2.camel@kernel.crashing.org>

On Fri, Nov 20, 2015 at 01:56:39PM +1100, Benjamin Herrenschmidt wrote:
> On Thu, 2015-11-19 at 23:38 +0000, David Woodhouse wrote:
> > 
> > I understand that POWER and other platforms don't currently have a
> > clean way to indicate that certain device don't have translation. And I
> > understand that we may end up with a *quirk* which ensures that the DMA
> > API does the right thing (i.e. nothing) in certain cases.
> > 
> > But we should *NOT* be involving the virtio device drivers in that
> > quirk, in any way. And putting a feature bit in the virtio device
> > itself doesn't seem at all sane either.
> > 
> > Bear in mind that qemu-system-x86_64 currently has the *same* problem
> > with assigned physical devices. It's claiming they're translated, and
> > they're not.
> 
> It's not that clear but yeah ... as I mentioned, I can't find a
> way to do that quirk that won't break when we want to actually use
> the iommu... 
> 
> Ben.

Yes, I am not at all sure we need a quirk for assigned devices.
Better teach QEMU to make iommu work for them.

-- 
MST