Cannot turn iommu on inside guest

[Daniel P. Berrangé]

On Tue, Feb 11, 2020 at 09:09:36AM -0500, Mauricio Tavares wrote:
> So I want to turn iommu support on in a guest (    <type arch='x86_64'
> machine='pc-q35-4.1'>hvm</type>). Per [1] I tried to add to devices
> either
> 
> <devices>
>   <iommu model='intel'>
>     <driver intremap='on'/>
>   </iommu>
> </devices>
> 
> or just a simpler
> 
> <devices>
>   <iommu model='intel /'>
> </devices>
> 
> However, when I try to boot it, it crashes with the following error
> message (stolen from /var/log/libvirt/qemu/testkvm.log):
> 
> 2020-02-11T05:13:27.278707Z qemu-system-x86_64: We need to set
> caching-mode=on for intel-iommu to enable device assignment with IOMMU
> protection.
> 2020-02-11 05:13:27.577+0000: shutting down, reason=crashed
> 
> that sounds like it wants me to do something like
> 
> qemu-system-x86_64 [...] -device intel-iommu,intremap=on,caching-mode=on[2]
> 
> using virsh. But
> 
> <devices>
>   <iommu model='intel'>
>     <driver intremap='on' caching-mode='on '/>

You need   caching_mode="on"  - notice underscore, not hypehn

>   </iommu>
> </devices>
> 
> is not only not mentioned in [1] but also fails

The correctly spelt caching_mode is mentioned there.

> 
> [root at testbox tests]# virsh edit testkvm
> error: XML document failed to validate against schema: Unable to
> validate doc against /usr/share/libvirt/schemas/domain.rng
> Extra element devices in interleave
> Element domain failed to validate content
> 
> Failed. Try again? [y,n,i,f,?]:
> 
> Can anyone show me what I am missing here?
> 
> [1] https://libvirt.org/formatdomain.html#elementsIommu
> [2] https://wiki.qemu.org/Features/VT-d#Command_Line_Example_2
> 
> 

Regards,
Daniel
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