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RE: [Xen-devel] [RFC][PATCH] 0/9 Populate-on-demand memory

Tian, Kevin wrote:
>> From: George Dunlap
>> Sent: Wednesday, December 24, 2008 10:43 PM
>>> Another tricky point could be with VT-d. If one guest page is used
>>> as DMA target before balloon driver is installed, and no early
>>> access on that page (like start-of-day scrubber), then PoD action
>>> will not be triggered... Not sure the possibility of such
>>> condition, but you may need to have some thought or guard on that.
>>> em... after more thinking, actually PoD pages may be alive even
>>> after balloon driver is installed. I guess before coming up a
>>> solution you may add a check on whether target domain has
>>> passthrough device to decide whether this feature is on on-the-fly. 
>> Hmm, I haven't looked at VT-d integration; it at least requires some
>> examination.  How are gfns translated to mfns for the VT-d hardware?
>> Does it use the hardware EPT tables?  Is the transaction re-startable
>> if we get an EPT fault and then fix the EPT table?
> there's a VT-d page table walked by VT-d engine, which is similar to
> EPT content. When device dma request is intercepted by VT-d engine,
> VT-d page table corresponding to that device is walked for valid
> mapping. Not like EPT which is restartable, VT-d page fault is just
> for log purpose since pci bus doesn't support I/O restart yet
> (although pcisig is looking at this possibility). That says, if we
> can't find a chance to trigger a cpu page fault before PoD page is
> used as dma target, either one should be disabled if both are
> configured. 
>> A second issue is with the emergency sweep: if a page which happens
>> to be zero ends up being the target of a DMA, we may get:
>> * Device request to write to gfn X, which translates to mfn Y.
>> * Demand-fault on gfn Z, with no pages in the cache.
>> * Emergency sweep scans through gfn space, finds that mfn Y is empty.
>> It replaces gfn X with a PoD entry, and puts mfn Y behind gfn Z.
>> * The request finishes.  Either the request then fails (because EPT
>> translation for gfn X is not valid anymore), or it silently succeeds
>> in writing to mfn Y, which is now behind gfn Z instead of gfn X.
> yes, this is also one issue. the request will fail since the dma
> address written to device is gfn, while X->Y mapping is cut off due
> to sweep. 
>> If we can't tell that there's an outstanding I/O on the page, then we
>> can't do an emergency sweep.  If we have some way of knowing that
>> there's *some* outstanding I/O to *some* page, we could pause the
>> guest until the I/O completes, then do the sweep.
> one possibility is to have a pv dma engine or virtual VT-d engine
> within guest, but that's another story.
>> At any rate, until we have that worked out, we should probably add
>> some "seatbelt" code to make sure that people don't use PoD for a
>> VT-d enabled domain.  I know absolutely nothing about the VT-d code;
>> could you either write a patch to do this check, or give me an idea
>> of the simplest thing to check?
> Weidong works on VT-d and could give comments on exact point
> to check.

You can simply check "iommu_enabled" to know whether IOMMU including VT-d and 
AMD IOMMU is used or not.


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