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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 27 Apr 2016 07:31:43 -0700
Message-ID: <CALCETrVkSSJbjoK8i7pLsSYR0o=Wy1UP-mrmn2uxYUd81g18dg@mail.gmail.com>
Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api
To: Joerg Roedel <joro@8bytes.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
        David Woodhouse <dwmw2@infradead.org>, Kevin Wolf <kwolf@redhat.com>,
        Wei Liu <wei.liu2@citrix.com>, Andy Lutomirski <luto@kernel.org>,
        qemu-block@nongnu.org, Christian Borntraeger <borntraeger@de.ibm.com>,
        Jason Wang <jasowang@redhat.com>,
        Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        "qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>,
        peterx@redhat.com,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Amit Shah <amit.shah@redhat.com>, iommu@lists.linux-foundation.org,
        Stefan Hajnoczi <stefanha@redhat.com>, kvm list <kvm@vger.kernel.org>,
        Cornelia Huck <cornelia.huck@de.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Anthony PERARD <anthony.perard@citrix.com>
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On Wed, Apr 27, 2016 at 7:23 AM, Joerg Roedel <joro@8bytes.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