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 15:15, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> On Fri, Jan 10, 2014 at 02:09:12PM +0000, Jan Beulich wrote:
>> >>> On 10.01.14 at 15:03, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
>> > On Fri, Jan 10, 2014 at 12:28:52PM +0000, Jan Beulich wrote:
>> >> >>> 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.
>> >> 
>> > 
>> > Do you have suggestion on how this comment should be phrased?
>> 
>> Just say "IOMMU" or "DMA remapping" instead of "VT-d".
>> 
> 
> OK, like this:
> 
> /* We cannot have PoD and PCI device assignment at the same time for HVM
>  * guest. It was reported that IOMMU cannot work with PoD enabled
>  * because it needs to populated entire page table for guest. To stay on
>  * the safe side, we disable PCI device assignment when PoD is enabled.
>  */

Sound fine to me. As does Ian's suggestion.

Jan


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