Re: [Xen-devel] [PATCH V2] amd iommu: re-enable iommu msi if dom0 disabled it

[Wei Wang <wei.wang2@xxxxxxx>]

Am 12.06.2012 18:43, schrieb Jan Beulich:
> > > > On 12.06.12 at 18:08, Andrew Cooper<andrew.cooper3@xxxxxxxxxx>  wrote:
> > On 12/06/12 16:13, Jan Beulich wrote:
> > > > > > On 12.06.12 at 14:02, Wei Wang<wei.wang2@xxxxxxx>  wrote:
> > > > I had attached a revised patch, please check it.
> > > While the patch technically looks better now, you didn't eliminate
> > > my objections to the approach you take, nor did you comment on
> > > the proposed alternative.
> > >
> > > A fundamental problem is that your IOMMUs show up as a "normal"
> > > PCI devices, breaking the separation between what is being
> > > managed by the hypervisor vs by the Dom0 kernel. (This even
> > > allows something as odd as passing through an IOMMU to a
> > > DomU, which would clearly upset the hypervisor.)
> > >
> > > > I found that the following Linux commit triggers this issue. It has been
> > > > included into 3.4 pv_ops.
> > > >
> > > > " commit a776c491ca5e38c26d9f66923ff574d041e747f4
> > > >     Author: Eric W. Biederman<ebiederm@xxxxxxxxxxxx>
> > > >     Date:   Mon Oct 17 11:46:06 2011 -0700
> > > >
> > > >     PCI: msi: Disable msi interrupts when we initialize a pci device "
> > > Thanks for locating this. As it stands, it is incomplete though
> > > anyway: If the kexec-ed kernel is one built without CONFIG_PCI_MSI,
> > > it won't have a means to suppress the "screaming" interrupts.
> > I feel that the correct solution would be for Xen to hide the PCI
> > devices from dom0.  Xen already hides the DMAR ACPI table (by turning it
> > to an XMAR table), and this would be the logical way to proceed now that
> > IOMMU internals are appearing as PCI devices.
That sounds absolutely good to me. thanks for the suggestion.

> That is precisely what I suggested in my response to the first
> version of this patch, and I'd also volunteer to look into putting
> together a first draft implementation if we sort of agree that
> this is the way to go.
Cool! thanks for doing that. Looking forward to it in Xen 4.2 since 
iommu msi is really broken with recent Linux dom0...
Thanks
Wei


> > ( A similar issue comes into play now that newer generations of CPUs are
> > exposing CPU internals as PCI devices )
> Indeed, good point. Might be a little tricky though to determine
> which one(s) they are, and still avoid conflicting with things like
> the EDAC drivers in Dom0.
>
> Jan


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