Re: [Xen-devel] [PATCH v2] libxl: disallow PCI device assignment for HVM guest when PoD is enabled

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-01-14 at 14:54 +0000, Andrew Cooper wrote:
> On 14/01/14 14:50, Ian Campbell wrote:
> > On Mon, 2014-01-13 at 11:52 +0000, Wei Liu wrote:
> >> This replicates a Xend behavior, see ec789523749 ("xend: Dis-allow
> >> device assignment if PoD is enabled.").
> >>
> >> This change is restricted to HVM guest, as only HVM is relevant in the
> >> counterpart in Xend. We're late in release cycle so the change should
> >> only do what's necessary. Probably we can revisit it if we need to do
> >> the same thing for PV guest in the future.
> >>
> >> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> >
> > Release hat: The risk here is of a false positive detecting whether PoD
> > would be used and therefore refusing to start a domain. However Wei
> > directed me earlier on to the code in setup_guest which sets
> > XENMEMF_populate_on_demand and I believe it is using the same logic.
> >
> > The benefit of this is that it will stop people starting a domain in an
> > invalid configuration -- but what is the downside here? Is it an
> > unhandled IOMMU fault or another host-fatal error? That would make the
> > argument for taking this patch pretty strong. On the other hand if the
> > failure were simply to kill this domain, that would be a less serious
> > issue and I'd be in two minds, mainly due to George not being here to
> > confirm that the pod_enabled logic is correct (although if he were here
> > I wouldn't be wrestling with this question at all ;-)).
> >
> > I'm leaning towards taking this fix, but I'd really like to know what
> > the current failure case looks like.
> >
> > Ian.
> 
> The answer is likely hardware specific.
> 
> An IOMMU fault (however handled by Xen) will result in a master abort on
> the DMA transaction for the PCI device which has suffered the fault. 
> That device can then do anything from continue blindly to issuing an NMI
> IOCK/SERR which will likely be fatal to the entire server.

Thanks, that tips me over into:
Release-ack: Ian Campbell

I applied, there was a reject against "libxl: Auto-assign NIC devids in
initiate_domain_create" which I fixed up.




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