Xen devel - Apr 2011 - [PATCH][VTD] force boot to fail if interrupt remapping cannot be enabled when iommu=force

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.

Signed-off-by: Allen Kay <allen.m.kay@intel.com>


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On 28/04/2011 00:42, "Kay, Allen M" <allen.m.kay@intel.com>
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.

 -- Keir
> Signed-off-by: Allen Kay <allen.m.kay@intel.com>


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On 04/28/11 19:05, Keir Fraser wrote:
> On 28/04/2011 00:42, "Kay, Allen M" <allen.m.kay@intel.com>
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?
Indeed, seems to me like a desired behavior.

j.
> As it is, this looks like a
> panic that is never realistically going to trigger.
> 
>  -- Keir
> 
>> Signed-off-by: Allen Kay <allen.m.kay@intel.com>
> 
> 
> 
> _______________________________________________
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> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel



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> From: Keir Fraser [mailto:keir.xen@gmail.com]
> Sent: Thursday, April 28, 2011 10:05 AM
> 
> On 28/04/2011 00:42, "Kay, Allen M" <allen.m.kay@intel.com>
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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