Re: [PATCH 04/12] pci-p2p: Clear ACS P2P flags for all client devices

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 4 Jan 2018 17:00:47 -0700
Logan Gunthorpe <logang@deltatee.com> wrote:

> On 04/01/18 03:35 PM, Alex Williamson wrote:
> > Yep, flipping these ACS bits invalidates any IOMMU groups that depend
> > on the isolation of that downstream port and I suspect also any peers
> > within the same PCI slot of that port and their downstream devices.  The
> > entire sub-hierarchy grouping needs to be re-evaluated.  This
> > potentially affects running devices that depend on that isolation, so
> > I'm not sure how that happens dynamically.  A boot option might be
> > easier.  Thanks,  
> 
> I don't see how this is the case in current kernel code. It appears to 
> only enable ACS globally if the IOMMU requests it.

IOMMU groups don't exist unless the IOMMU is enabled and x86 and ARM
both request ACS be enabled if an IOMMU is present, so I'm not sure
what you're getting at here.  Also, in reply to your other email, if
the IOMMU is enabled, every device handled by the IOMMU is a member of
an IOMMU group, see struct device.iommu_group.  There's an
iommu_group_get() accessor to get a reference to it.
 
> I also don't see how turning off ACS isolation for a specific device is 
> going to hurt anything. The IOMMU should still be able to keep going on 
> unaware that anything has changed. The only worry is that a security 
> hole may now be created if a user was relying on the isolation between 
> two devices that are in different VMs or something. However, if a user 
> was relying on this, they probably shouldn't have turned on P2P in the 
> first place.

That's exactly what IOMMU groups represent, the smallest set of devices
which have DMA isolation from other devices.  By poking this hole, the
IOMMU group is invalid.  We cannot turn off ACS only for a specific
device, in order to enable p2p it needs to be disabled at every
downstream port between the devices where we want to enable p2p.
Depending on the topology, that could mean we're also enabling p2p for
unrelated devices.  Those unrelated devices might be in active use and
the p2p IOVAs now have a different destination which is no longer IOMMU
translated.
 
> We started with a fairly unintelligent choice to simply disable ACS on 
> any kernel that had CONFIG_PCI_P2P set. However, this did not seem like 
> a good idea going forward. Instead, we now selectively disable the ACS 
> bit only on the downstream ports that are involved in P2P transactions. 
> This seems like the safest choice and still allows people to (carefully) 
> use P2P adjacent to other devices that need to be isolated.

I don't see that the code is doing much checking that adjacent devices
are also affected by the p2p change and of course the IOMMU group is
entirely invalid once the p2p holes start getting poked.

> I don't think anyone wants another boot option that must be set in order 
> to use this functionality (and only some hardware would require this). 
> That's just a huge pain for users.

No, but nor do we need IOMMU groups that no longer represent what
they're intended to describe or runtime, unchecked routing changes
through the topology for devices that might already be using
conflicting IOVA ranges.  Maybe soft hotplugs are another possibility,
designate a sub-hierarchy to be removed and re-scanned with ACS
disabled.  Otherwise it seems like disabling and re-enabling ACS needs
to also handle merging and splitting groups dynamically.  Thanks,

Alex