Re: [Xen-devel] [PATCH v4 08/16] xen/mm: Drop the parameter mfn from populate_pt_range

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.02.18 at 17:55, <julien.grall@xxxxxxx> wrote:
> On 22/02/18 16:51, Wei Liu wrote:
>> On Thu, Feb 22, 2018 at 04:40:04PM +0000, Julien Grall wrote:
>>> On 22/02/18 16:35, Wei Liu wrote:
>>>> On Wed, Feb 21, 2018 at 02:02:51PM +0000, Julien Grall wrote:
>>>>> The function populate_pt_range is used to populate in advance the
>>>>> page-table but it will not do the actual mapping. So passing the MFN in
>>>>> parameter is pointless. Note that the only caller pass 0...
>>>>>
>>>>> At the same time replace 0 by INVALID_MFN to make clear the MFN is
>>>>> invalid.
>>>>>
>>>>
>>>> The mfn parameter is the first mfn of a consecutive nr MFNs passed to
>>>> map_pages_to_xen. Putting INVALID_MFN isn't helping -- the value written
>>>> to page table(s) will wrap around to 0.
>>>>
>>>> And I think starting from 0 to avoid overflow is probably a better
>>>> behaviour. If you really want to make sure all entries are filled with
>>>> INVALID_MFN you should call map_pages_to_xen for nr times with each
>>>> page.
>>>
>>> I am not sure to understand this. From its name, populate_pt_range should
>>> only create the intermediate tables. The leaf entry will stay invalid. So
>>> how the value of mfn matters? Is it because the code is written in a such
>>> way that passing INVALID_MFN will result to undefined behavior?
>> 
>> Right, that's what I meant. It doesn't matter whether you use 0 or
>> INVALID_MFN.
>> 
>> Unsigned integer overflow is not UB in C, so passing INVALID_MFN is
>> safe.
>> 
>> But your intention seemed to be filling all entries with INVALID_MFN to
>> aid debugging, so the function doesn't do what I think you wanted it to
>> do. It could be I misunderstood your intention.
> 
> That was not my intention. I replaced 0 by INVALID_MFN because from the 
> name you know the MFN is invalid. 0 could potentially be valid (at least 
> on Arm) and make the code confusing to understand.
> 
> I can make it clearer in the commit message.

I don't think that'll be much better; I agree with Wei that you
don't want the wrapping behavior here. What you want to do
is skip the increments in x86's map_pages_to_xen() when
mfn is INVALID_MFN. Granted this should have been done
before (so that there wouldn't have been incrementing from
zero), but as you say MFN 0 isn't fundamentally invalid (albeit
on x86 we almost make it invalid).

As to your earlier argument - please don't forget that on x86
the function still fills all leaf entries in the range, just that they
all will be non-present ones.

Jan


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