Re: kvm PCI assignment & VFIO ramblings

Alex Williamson
2011-08-05T15:10:09Z

On Fri, 2011-08-05 at 20:42 +1000, Benjamin Herrenschmidt wrote:
> Right. In fact to try to clarify the problem for everybody, I think we
> can distinguish two different classes of "constraints" that can
> influence the grouping of devices:
>
> 1- Hard constraints. These are typically devices using the same RID or
> where the RID cannot be reliably guaranteed (the later is the case with
> some PCIe-PCIX bridges which will take ownership of "some" transactions
> such as split but not all). Devices like that must be in the same
> domain. This is where PowerPC adds to what x86 does today the concept
> that the domains are pre-existing, since we use the RID for error
> isolation & MMIO segmenting as well. so we need to create those domains
> at boot time.
>
> 2- Softer constraints. Those constraints derive from the fact that not
> applying them risks enabling the guest to create side effects outside of
> its "sandbox". To some extent, there can be "degrees" of badness between
> the various things that can cause such constraints. Examples are shared
> LSIs (since trusting DisINTx can be chancy, see earlier discussions),
> potentially any set of functions in the same device can be problematic
> due to the possibility to get backdoor access to the BARs etc...

This is what I've been trying to get to, hardware constraints vs system
policy constraints.

> Now, what I derive from the discussion we've had so far, is that we need
> to find a proper fix for #1, but Alex and Avi seem to prefer that #2
> remains a matter of libvirt/user doing the right thing (basically
> keeping a loaded gun aimed at the user's foot with a very very very
> sweet trigger but heh, let's not start a flamewar here :-)

Doesn't your own uncertainty of whether or not to allow this lead to the
same conclusion, that it belongs in userspace policy? I don't think we
want to make white lists of which devices we trust to do DisINTx
correctly part of the kernel interface, do we? Thanks,

Alex