From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH v4 0/4] virtio: Clean up scatterlists and use the DMA API
Date: Wed, 03 Sep 2014 17:11:45 +0200
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To: Andy Lutomirski <luto@amacapital.net>
Cc: "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>, Benjamin Herrenschmidt <benh@au1.ibm.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, "Michael S. Tsirkin" <mst@redhat.com>, Linux Virtualization <virtualization@lists.linux-foundation.org>, Christian Borntraeger <borntraeger@de.ibm.com>, "linux390@de.ibm.com" <linux390@de.ibm.com>
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Il 03/09/2014 17:07, Andy Lutomirski ha scritto:
>> > Yes, only for virtio---but for x86 I think it should be off by default,
>> > even if that means virtio+IOMMU requires a new kernel.
> Just to clarify: is "it" the direct-ram-access property?  If so, I
> think I might agree.

Yes.

> Alternatively, could QEMU easily teach the IOMMU code to generate the
> ACPI tables such that virtio-pci devices aren't advertised as living
> behind the IOMMU?  This would work both with and without my patches.
> On the other hand, maybe this gets complicated when hotplug is
> involved.

That could be possible.  For hot-plug, you can simply forbid hotplugging
with direct-ram-access=on.  If you want hotplug, then we should add the
direct-ram-access property to PCI bridges too (with the same limitation
on hotplug).  All devices under such a bridge would be outside the
IOMMU, including virtio devices with direct-ram-access=off.

Paolo