Re: [Xen-devel] PCI-passthrough for 32 bit guests and high MMIO addresses

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.11.14 at 16:17, <JGross@xxxxxxxx> wrote:
> On 11/21/2014 03:54 PM, Jan Beulich wrote:
>>>>> On 21.11.14 at 15:39, <JGross@xxxxxxxx> 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?

For MMIO, why not? All that counts there is the page table entry
format.

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

MMIO isn't subject to saving/restoring.

Jan


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