Re: [Qemu-devel] kvm PCI assignment & VFIO ramblings

[David Gibson]

On Thu, Aug 25, 2011 at 08:25:45AM -0500, Alexander Graf wrote:
> 
> On 25.08.2011, at 07:31, Roedel, Joerg wrote:
> 
> > On Wed, Aug 24, 2011 at 11:07:46AM -0400, Alex Williamson wrote:
> >> On Wed, 2011-08-24 at 10:52 +0200, Roedel, Joerg wrote:
> > 
> 
> [...]
> 
> >> We need to try the polite method of attempting to hot unplug the device
> >> from qemu first, which the current vfio code already implements.  We can
> >> then escalate if it doesn't respond.  The current code calls abort in
> >> qemu if the guest doesn't respond, but I agree we should also be
> >> enforcing this at the kernel interface.  I think the problem with the
> >> hard-unplug is that we don't have a good revoke mechanism for the mmio
> >> mmaps.
> > 
> > For mmio we could stop the guest and replace the mmio region with a
> > region that is filled with 0xff, no?
> 
> Sure, but that happens in user space. The question is how does
> kernel space enforce an MMIO region to not be mapped after the
> hotplug event occured? Keep in mind that user space is pretty much
> untrusted here - it doesn't have to be QEMU. It could just as well
> be a generic user space driver. And that can just ignore hotplug
> events.

We're saying you hard yank the mapping from the userspace process.
That is, you invalidate all its PTEs mapping the MMIO space, and don't
let it fault them back in.

As I see it there are two options: (a) make subsequent accesses from
userspace or the guest result in either a SIGBUS that userspace must
either deal with or die, or (b) replace the mapping with a dummy RO
mapping containing 0xff, with any trapped writes emulated as nops.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson