Re: [Xen-devel] [PATCH][RFC] Support more Capability Structures andDevice Specific

[Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>]

> I think it is fine to have a passthrough option which doesn't properly
> protect the host from the guest - this is a useful setup in many
> situations.  But it should not be enabled by default, surely ?

Agreed entirely. Note also that some implementations of an IOMMU will not
save you as they don't fence between individual PCI devices (PCIE is
obviously a bit easier). Not fencing between devices allows you for
example to use a fairly flexible SCSI controller to reprogram another
device. 

In the general case there are also some really nasty dirty attacks you
can't stop with an IOMMU one of which is to reflash the BIOS of the
graphics card to which you were given unrestricted access so that you
compromise the entire system next boot. These attacks appear well
understood except by IOMMU marketing people ;)

IOMMU is great for system correctness and flexibility, using it for
safely providing hardware direct access is a very very hairy business with
a complex device.

Alan

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