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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-01-10 at 14:03 +0000, Wei Liu 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?

We cannot have PoD and PCI device assignment at the same to for HVM
guests. This is because the IOMMU support in Xen cannot currently cope
with faults due to pages which are not present.

?



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