[Xen-devel] RE: [PATCH][VTD] force boot to fail if interrupt remapping cannot be enabled when iommu=force

["Cihula, Joseph" <joseph.cihula@xxxxxxxxx>]

> From: Keir Fraser [mailto:keir.xen@xxxxxxxxx]
> Sent: Thursday, April 28, 2011 10:05 AM
> 
> On 28/04/2011 00:42, "Kay, Allen M" <allen.m.kay@xxxxxxxxx> wrote:
> 
> > Force Xen boot to fail if interrupt remapping fails to enable and the
> > following are true: iommu=force is set as xen boot parameter, VT-d
> > engine HW is interrupt remapping capable, DMAR_INTR_REMAP bit is set in 
> > DMAR flags.
> > This forces iommu=force boot instances has interrupt remapping enabled
> > if HW and BIOS supports it.
> 
> If HW and BIOS support it, why would it fail to be enabled? This doesn't look 
> like a particularly
> useful panic() path. If interrupt remapping is so important, perhaps 
> iommu=force should
> unconditionally require it, and panic in its absence regardless of platform 
> features? As it is,
> this looks like a panic that is never realistically going to trigger.

There are ways that malicious SW, running before the TXT launch of Xen, can 
alter the DMAR ACPI tables such that Xen will fail to enable interrupt 
remapping (IR).

Since not all platforms support IR, we certainly don't want to fail Xen on 
those platforms.  We simply want to ensure that if the HW does support IR that 
Xen can't be "tricked" into not using it.

Joe

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