Re: [Xen-devel] [PATCH] libxl: disallow PCI device assignment for HVM guest when PoD is enabled

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.01.14 at 13:16, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 10/01/2014 11:46, Wei Liu wrote:
>> On Fri, Jan 10, 2014 at 11:39:51AM +0000, Jan Beulich wrote:
>>>>>> On 10.01.14 at 12:27, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
>>>> +    /* We cannot have PoD and PCI device assignment at the same time
>>>> +     * for HVM guest. It was reported that VT-d engine cannot
>>> There's still a "VT-d" left in here...
>>>
>> I mentioned AMD as well. Was trying to clarify things a bit more...
> 
> Use "IOMMU"/"PCI Passthrough"/etc as appropriate, which are vendor neutral.
> 
> There is no way for for PoD (or Paging for that matter) to work in
> combination with PCIPassthrough, as you need all the backing RAM for all
> gfns to exist to receive DMA.

That's going a little too far: If IOMMU faults were recoverable, dealing
with non-present pages would become possible (with other caveats
of course). So this is not a fundamental attribute of IOMMUs, but
there doesn't seem to be any reason to believe that the current
model would change any time soon for either of the vendors.

Jan


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