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 09:47:07 +0200
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To: Benjamin Herrenschmidt <benh@au1.ibm.com>, Andy Lutomirski <luto@amacapital.net>
Cc: "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>, 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 02:25, Benjamin Herrenschmidt ha scritto:
> > But there aren't any ACPI systems with both virtio-pci and IOMMUs,
> > right?  So we could say that, henceforth, ACPI systems must declare
> > whether virtio-pci devices live behind IOMMUs without breaking
> > backward compatibility.
> 
> I don't know for sure whether that's the case and whether we can rely on
> that not happening, we'll need x86 folks opinion here.

IOMMU support for x86 is going to go in this week.

However, it is and likely will remain niche enough that I don't really
care about performance loss from IOMMU support.  If you enable it, you
want it.

So from the QEMU point of view we can simply add the direct-ram-access
property, and have the pseries machine turn it on by default (while
other machines can leave it off by default---they have no IOMMU and thus
no performance cost).

Paolo