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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Feb 22, 2018 at 04:40:04PM +0000, Julien Grall wrote:
> Hi Wei,
> 
> 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.

Wei.

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