Re: [Xen-devel] [driver question] IOMMU concept in Xen

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Maciek <gotoc64@xxxxxxxxx> 10/10/16 10:17 PM >>>

First of all - please don't top post.

>In driver initialization we can call sriov_configure to turn on SRIOV 
>and enable VFs.
>To be able to run on VM using VF we need IOMMU enabled (currently we 
>recommend our
>customers to add parameter to kernel intel_iommu=on).
>But because we have many cases when people were trying to run driver 
>without iommu,
>caused fail, that VFs were visible but driver not working we decided to 
>handle inside sriov_configure
>call to handle this case by using simple check by iommu_present.
>That solve our issue till we moved to Xen.
>As I pointed out on Xen we didn't see iommu on Dom0, and that cause our 
>check to don't enable sriov.
>After we removed this check driver works perfectly fine, but we are in 
>the same place.
>So my intention is to leave this check in case of other hypervisor, and 
>also meet Xen requirements about iommu handling.

Hmm, as indicated the Dom0 kernel doesn't and isn't supposed to know
of IOMMUs and their enabled state, so there's a conceptual problem here.
I'm afraid your users will need to learn not to disable the IOMMU on Xen (it's
enabled by default). The tool stack (which normally runs in Dom0 user
space) can know, and iirc it also prevents pass-through of devices when
the IOMMU is not enabled. Wouldn't the resulting error message not be
sufficient for your purposes?

Jan


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