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 14:11:01 +0200
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To: Benjamin Herrenschmidt <benh@au1.ibm.com>
Cc: "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, "Michael S. Tsirkin" <mst@redhat.com>, Andy Lutomirski <luto@amacapital.net>, Christian Borntraeger <borntraeger@de.ibm.com>, "linux390@de.ibm.com" <linux390@de.ibm.com>, Linux Virtualization <virtualization@lists.linux-foundation.org>
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Il 03/09/2014 10:05, Benjamin Herrenschmidt ha scritto:
> On Wed, 2014-09-03 at 09:47 +0200, Paolo Bonzini wrote:
>>
>> IOMMU support for x86 is going to go in this week.
> 
> But won't that break virtio on x86 ? Or will virtio continue bypassing
> it ? IE, the guest side virtio doesn't expect an IOMMU and doesn't call
> the dma mappings ops.
> 
>> 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).
> 
> Well, it's only for virtio and should be on by default on x86 as well if
> an iommu is installed no ?

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.

Paolo