Re: [Xen-devel] [PATCH v3] x86/p2m: use large pages for MMIO mappings

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.01.16 at 11:09, <ian.campbell@xxxxxxxxxx> wrote:
> On Thu, 2016-01-14 at 03:04 -0700, Jan Beulich wrote:
>> - ARM side unimplemented (and hence libxc for now made cope with both
>>   models),
> 
> So, one model is the one described in the commit message:
> 
>> - zero (success, everything done)
>> - positive (success, this many done, more to do: re-invoke)
>> - negative (error)
> 
> What is the other one? I'd expect ARM to already implement a subset of this
> (i.e. 0 or negative, perhaps with a subset of the possible errno values), 
> which I'd then expect libxc to just cope with without it constituting a 
> second model.

Well, first of all ARM doesn't get switched away from the current
model (yet), i.e. returning -E2BIG out of do_domctl(). And then
the difference between what the patch implements and what the
non-commit message comment describes is how errors get handled:
The patch makes a negative error value returned upon error, with
the caller having no way to tell at what point the error occurred
(and with a best effort undo in the case of "map"). The described
alternative would return the number of succeeded entries unless
an error occurred on the very first MFN, without any attempt to
undo the part that was done successfully. I.e. it would leave it
to the caller to decide what to do, and whether/when to roll back.

Jan


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