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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 12/06/12 16:13, Jan Beulich wrote:
>>>> On 12.06.12 at 14:02, Wei Wang <wei.wang2@amd.com> 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@xmission.com>
>>    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.
>
> Jan

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.

( A similar issue comes into play now that newer generations of CPUs are
exposing CPU internals as PCI devices )

~Andrew

>
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-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com