Re: PCI-passthrough for 32 bit guests and high MMIO addresses

[Juergen Gross <jgross@suse.com>]

On 11/21/2014 03:54 PM, Jan Beulich wrote:
>>>> On 21.11.14 at 15:39, <JGross@suse.com> wrote:
>> Trying to do PCI-passthrough with a 32-bit pv-domain I passed the
>> wrong device to the domain. The MMIO address was too large for a
>> MFN of a 32-bit system (it was 380003200000-3800036fffff).
>>
>> Instead of rejecting the operation Xen tried to perform it resulting
>> in a (quite understandable) failure in the domU.
>>
>> I think either the hypervisor or the tools should refuse to do
>> PCI-passthrough in this case.
>
> What's wrong with this large an address? 32-bit PV uses PAE, i.e.
> can map them. If the kernel isn't capable of that that's not
> something to make Xen (or the tools) refuse such assignments. I
> would only see an issue if a hypercall interface involved here isn't
> using wide enough fields (but these addresses should be read
> from the BARs, i.e. no hypercall involved).

The MFN format is part of the pv-ABI. And a MFN of a 32-bit pv-guest is
only 32 bits (even if don't take the invalid bit into account).

Should a pv-guest really be capable to map an address outside it's
accessible MFN-range?

Are the tools capable of processing such a mapping in case of saving the
domain?


Juergen