Re: [Xen-users] ARM: "xen_add_mach_to_phys_entry: cannot add ... already exists and panics"

[David Vrabel <david.vrabel@citrix.com>]

On 04/07/14 15:12, Stefano Stabellini wrote:
> On Fri, 4 Jul 2014, David Vrabel wrote:
>> On 03/07/14 18:47, Stefano Stabellini wrote:
>>>
>>> At the moment I would like a way to disable grant mappings and go back
>>> to grant copies on demand. Maybe we could have a feature flag to change
>>> the behaviour of the network backend?
>>
>> You must fix the ARM code to handle this properly.
>>
>> blkback also uses grant maps and depending on the frontend blkback may
>> grant map the same machine frame multiple time.  We have see frontends
>> that send such requests.
> 
> Indeed, it must be fixed properly. The workaround of disabling grant
> mappings would be just to buy us some time to come up with the fix.

It's an expensive workaround though.

>> I can't remember how the ARM code works.  Where is the problematic m2p
>> lookup?
> 
> arch/arm/xen/p2m.c
> 
> 
>> Perhaps this could be avoided for foreign frames?
> 
> Unfortunately no. The whole point of p2m.c is to be able to translate
> foreign frames for dma operations.

This is a p2m lookup though, which is fine, yes?  Where, specifically is
a mfn_to_pfn() lookup needed for a foreign MFN.

FWIW, On x86, mfn_to_pfn() on a foreign MFN will return an invalid value
(unless the gntdev device made the mapping and the m2p_override is used).

David