RE: [Xen-devel] pciback: question about the permissive flag

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>]

> I think the fear was that there could be class- or device-specific config
> registers that we wouldn't know how to handle, and which could have
> unexpected effects if they are passed through naively. Concrete examples
> were never given, and this was all pre-vtd so as you say pass-through of a
> DMA-capable device was insecure anyway. I've always thought the permissive
> flag stuff was pretty useless, and I always suggest people to enable the
> permissive flag.

There are some devices (typically integrated ones, e.g. igfx) that use PCI 
config space in nasty ways, such as to describe additional BARs, or to trigger 
SMIs. Allowing free access to these seems dangerous. 

Ian

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