Re: [TESTDAY] PV / HVM pass-through works when IOMMU present; weird failures when not

["Jan Beulich" <JBeulich@suse.com>]

>>> On 28.06.13 at 17:37, George Dunlap <George.Dunlap@eu.citrix.com> wrote:
> - For HVM guests, the only user-visible indication tha the IOMMU has
> been disabled is the following error message on the command-line:
> 
> # xl pci-attach h0 07:00.0
> libxl: error: libxl_pci.c:949:do_pci_add: xc_assign_device failed
> 
> However, the device itself ends up passed-through to the guest anyway;
> the guest seems to be able to see it and interact with it normally.
> This is particularly scary, as in theory this should not be possible
> without a working IOMMU.
> 
> I don't think this is a blocker for 4.3, but we should definitely
> release note it, and for 4.4 add a check to see if there is a
> functioning IOMMU and only add a device if there's an override set.

To me this very much looks like a security problem (which I
think we should fix asap).

As I tried this the other day with a boot time assignment, and
it prevented the guest from booting (which is how it should be)
- are you also seeing the guest happily using such device when
assigned via guest config file?

Knowing that may hint at where to look for the actual problem.

Also, I can't really see how the guest would be able to interact
with a half way assigned device properly - I could imagine you
being able to look at its config space, and perhaps load the
driver, but I can't see an I/O to succeed, at least not as long
as any bus mastering is being used the device (this ought to
crash this guest, another guest, or the host, or deliver corrupt
data). Purely port based I/O would likely work, but other than
serial cards I can't think of many things that would do so.

Jan