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Re: [PATCH v4 0/6] virtio core DMA API conversion

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On 09/11/2015 13:15, Michael S. Tsirkin wrote:
> Well that's not exactly true. I think we would like to make
> it possible to put virtio devices behind an IOMMU on x86,
> but if this means existing guests break, then many people won't be able
> to use this option: having to find out which kernel version your guest
> is running is a significant burden.
> 
> So on the host side, we need to detect guests that
> don't program the IOMMU and make QEMU ignore it.
> I think we need to figure out a way to do this
> before we commit to the guest change.

What is the usecase for putting virtio devices behind an IOMMU, apart from:

1) "because you can"

2) using VFIO within the guest

?  Case 1 can be ignored, and in case 2 the guest will do the right thing.

> Additionally, IOMMU overhead is very high when running within the VM.
> So for uses such as VFIO, we'd like a way to make something like
> iommu-pt the default.

That's not something that the kernel cares about.  It's just a
configuration issue.

Paolo
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Re: [PATCH v4 0/6] virtio core DMA API conversion "Michael S. Tsirkin" <mst@redhat.com> - 2015-11-09 13:20 +0100
  Re: [PATCH v4 0/6] virtio core DMA API conversion Paolo Bonzini <pbonzini@redhat.com> - 2015-11-09 13:30 +0100
  Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 00:30 +0100
    Re: [PATCH v4 0/6] virtio core DMA API conversion Andy Lutomirski <luto@amacapital.net> - 2015-11-10 01:50 +0100
      Re: [PATCH v4 0/6] virtio core DMA API conversion Andy Lutomirski <luto@amacapital.net> - 2015-11-10 03:20 +0100
        Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 06:30 +0100
          Re: [PATCH v4 0/6] virtio core DMA API conversion Andy Lutomirski <luto@amacapital.net> - 2015-11-10 06:40 +0100
            Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 11:40 +0100
              Re: [PATCH v4 0/6] virtio core DMA API conversion "Michael S. Tsirkin" <mst@redhat.com> - 2015-11-10 13:50 +0100
                Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 21:40 +0100
              Re: [PATCH v4 0/6] virtio core DMA API conversion Andy Lutomirski <luto@amacapital.net> - 2015-11-10 20:00 +0100
                Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 23:30 +0100
                Re: [PATCH v4 0/6] virtio core DMA API conversion Andy Lutomirski <luto@amacapital.net> - 2015-11-11 00:50 +0100
                Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-11 01:50 +0100
                Re: [PATCH v4 0/6] virtio core DMA API conversion Andy Lutomirski <luto@amacapital.net> - 2015-11-11 05:50 +0100
                Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-11 06:10 +0100
        Re: [PATCH v4 0/6] virtio core DMA API conversion Andy Lutomirski <luto@amacapital.net> - 2015-11-10 06:40 +0100
        Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 07:30 +0100
        Re: [PATCH v4 0/6] virtio core DMA API conversion Jan Kiszka <jan.kiszka@siemens.com> - 2015-11-10 09:20 +0100
      Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 04:10 +0100
        Re: [PATCH v4 0/6] virtio core DMA API conversion Knut Omang <knut.omang@oracle.com> - 2015-11-10 10:50 +0100
          Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 11:50 +0100
        Re: [PATCH v4 0/6] virtio core DMA API conversion Joerg Roedel <jroedel@suse.de> - 2015-11-10 11:30 +0100
          Re: [PATCH v4 0/6] virtio core DMA API conversion Benjamin Herrenschmidt <benh@kernel.crashing.org> - 2015-11-10 20:40 +0100

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