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

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Date:	Wed, 27 Apr 2016 08:10:42 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Joerg Roedel <joro@...tes.org>,
	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 Developers" <qemu-devel@...gnu.org>,
	peterx@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Amit Shah <amit.shah@...hat.com>,
	iommu@...ts.linux-foundation.org,
	Stefan Hajnoczi <stefanha@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Linux Virtualization <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 7:54 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Wed, Apr 27, 2016 at 07:43:07AM -0700, Andy Lutomirski wrote:
>> On Wed, Apr 27, 2016 at 7:38 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
>> > On Wed, Apr 27, 2016 at 07:31:43AM -0700, Andy Lutomirski wrote:
>> >> On Wed, Apr 27, 2016 at 7:23 AM, Joerg Roedel <joro@...tes.org> 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. So any fix here belongs
>> >> > into the platform/iommu code too and not into some driver.
>> >> >
>> >> >> 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.
>> >> >
>> >>
>> >> Except on PPC and SPARC.  As far as I know, those are the only
>> >> problematic platforms.
>> >>
>> >> Is it too late to *disable* QEMU's q35-iommu thingy until it can be
>> >> fixed to report correct data in the DMAR tables?
>> >>
>> >> --Andy
>> >
>> > Meaning virtio or assigned devices?
>> > For virtio - it's way too late since these are working configurations.
>> > For assigned devices - they don't work on x86 so it doesn't have
>> > to be disabled, it's safe to ignore.
>>
>> I mean actually prevent QEMU from running in q35-iommu mode with any
>> virtio devices attached or maybe even turn off q35-iommu mode entirely
>> [1].  Doesn't it require that the user literally pass the word
>> "experimental" into QEMU right now?  It did at some point IIRC.
>>
>> The reason I'm asking is that, other than q35-iommu, QEMU's virtio
>> devices *don't* bypass the IOMMU except on PPC and SPARC, simply
>> because there is no other configuration AFAICT that has virtio and and
>> IOMMU.  So maybe the right solution is to fix q35-iommu to use DMAR
>> correctly (thus breaking q35-iommu users with older guest kernels,
>> which hopefully don't actually exist) and to come up with a PPC- and
>> SPARC-specific solution, or maybe OpenFirmware-specific solution, to
>> handle PPC and SPARC down the road.
>>
>> [1] I'm pretty sure I emailed the QEMU list before q35-iommu ever
>> showed up in a release asking the QEMU team to please not do that
>> until this issue was resolved.  Sadly, that email was ignored :(
>>
>> --Andy
>
> Sorry, I didn't make myself clear.
> Point is, QEMU is not the only virtio implementation out there.
> So we can't know no virtio implementations have an IOMMU as long as
> linux supports this IOMMU.
> virtio always used physical addresses since it was born and if it
> changes that it must do this in a way that does not break existing
> users.

Is there any non-QEMU virtio implementation can provide an
IOMMU-bypassing virtio device on a platform that has a nontrivial
IOMMU?

--Andy