[Bug 81841] amd-iommu: kernel BUG & lockup after shutting down KVM guest using PCI passthrough/PCIe bridge

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=81841

--- Comment #9 from Alex Williamson <alex.williamson@redhat.com> ---
(In reply to Marti Raudsepp from comment #8)
> (In reply to Alex Williamson from comment #7)
> > > There are some proposed workarounds on the web
> > None of these remotely address the issue.
> 
> I see. This page claims so: http://www.ovirt.org/Features/hostdev_passthrough

Sorry, it's wrong.

> > there are quirks for the following AMD southbridge components
> 
> Nope, mine are 1022:780b, 1022:780c, 1022:780d, 1022:780e, 1022:780f,
> 1022:7809
> 
> > If your bridge does not match these, then AMD will need to confirm whether
> > isolation is provided between your devices.
> 
> How would I go about confirming that? What are the chances that they care,
> and provide accurate information to a random person?

AMD would need to confirm it.  IOMMU groups are based on hardware advertised
isolation via the PCIe ACS capability.  Without this, or a device specific
quirk to take its place, IOMMU groups must assume that peer-to-peer between
functions of a multi-function device is possible and therefore that the devices
are not isolated.  Chances are that this new chipset in your system is taking
the exact same ASICs that were deemed not to do peer-to-peer on previous
chipsets, but we need that confirmation from AMD.  Alex Deucher (see
MAINTAINERS) may have contacts available that can make that statement.

> > There is an ACS override patch
> 
> I already ran across it...
> https://bugzilla.redhat.com/show_bug.cgi?id=1113399
> Would I be any worse off using this, compared to the old kvm pci-assign
> method?

I think the path forward is to get confirmation from AMD that these function
are isolated from each other and add quirks to the kernel.  Then you won't have
the device dependencies in vfio-pci.  The override patch allows you to do that
with just a kernel boot parameter.  There's no gurantee that pci-assign will
ever be fixed since it's being phased out.

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