Re: [Xen-devel] [PATCH v2] iommu: leave IOMMU enabled by default during kexec crash transition

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.02.19 at 23:08, <igor.druzhinin@xxxxxxxxxx> wrote:
> It's unsafe to disable IOMMU on a live system which is the case
> if we're crashing since remapping hardware doesn't usually know what
> to do with ongoing bus transactions and frequently raises NMI/MCE/SMI,
> etc. (depends on the firmware configuration) to signal these abnormalities.
> This, in turn, doesn't play well with kexec transition process as there is
> no handling available at the moment for this kind of events resulting
> in failures to enter the kernel.
> 
> Modern Linux kernels taught to copy all the necessary DMAR/IR tables
> following kexec from the previous kernel (Xen in our case) - so it's
> currently normal to keep IOMMU enabled. It might require minor changes to
> kdump command line that enables IOMMU drivers (e.g. intel_iommu=on /
> intremap=on) but recent kernels don't require any additional changes for
> the transition to be transparent.
> 
> A fallback option is still left for compatibility with ancient crash
> kernels which didn't like to have IOMMU active under their feet on boot.
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>

Based on the subsequent discussion
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan



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