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Date: Thu, 27 Sep 2012 12:22:41 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [big lock] Discussion about the convention of
 device's DMA each other after breaking down biglock
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To: Avi Kivity <avi@redhat.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, liu ping fan <qemulist@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Il 27/09/2012 12:08, Avi Kivity ha scritto:
>>>>> virtio-net is per-spec not going through the iommu.
>>> >> What's that smell?  A feature bit?
>> > 
>> > Why is it a bad thing that it does not go through the iommu?
> You can't assign the device to nested guests or to guest userspace via
> vfio.  Some guests may want to use the iommu even in the guest kernel
> context.

All good points.

However, using the iommu means you cannot use either vhost-net or (at
least for now) per-device lock.  So it closes some doors on performance
improvements... I can imagine pSeries guys prefer to keep no iommu in
virtio devices.

Paolo