From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [Xen-users] ARM: "xen_add_mach_to_phys_entry:
 cannot add ... already exists and panics"
Date: Fri, 4 Jul 2014 15:23:33 +0100
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To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>, David Vrabel <david.vrabel@citrix.com>
Cc: xen-devel@lists.xensource.com, "Wei Liu (Intern)" <wei.liu2@citrix.com>, Ian Campbell <Ian.Campbell@citrix.com>, Julien Grall <julien.grall@citrix.com>, Stefano Stabellini <stefano.stabellini@citrix.com>, Zoltan Kiss <zoltan.kiss@citrix.com>, xen-users@lists.xen.org, Denis Schneider <v1ne2go@gmail.com>
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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