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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.
 */

Wei.

> Jan

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