Re: [Xen-devel] [PATCH 8/8] x86/EPT: IOMMU snoop capability should not affect memory type selection

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.14 at 16:12, <tim@xxxxxxx> wrote:
> At 15:37 +0000 on 26 Mar (1395844658), Jan Beulich wrote:
>> This capability solely makes a statement on cache coherency guarantees
>> by the IOMMU. It does specifically not imply any further guarantees
>> implied by certain memory types (cachability, ordering).
> 
> Can you give some examples of what this is protecting against?
> 
> Cachability is irrelevant unless there's some other form of direct
> access that's not covered by the IOMMU, and x86 ordering is pretty
> strict.

What the IOMMU gets to see already depends on cachability: Especially
for write buffers (WC) I don't think cache coherence really matters.

And my understanding of "snoop" also means that stuff held in caches
(WB) may not become visible as needed when some RAM page is
shared between CPU and some device (namely a GPU): The IOMMU
can certainly snoop any accesses that hit the bus, but I don't think it
can enforce write-back to happen for an area that's marked WB but
was supposed to be UC. But maybe I'm wrong with this...

Jan


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