Re: [Xen-devel] [PATCH] xen/arm: skip first page when RAM starts at 0x0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.04.19 at 01:47, <sstabellini@xxxxxxxxxx> wrote:
> The other change to nr_pdxs is less obvious. It is clear that nr_pdxs is
> calculated wrongly in this case (memory 0-0x80000000,
> 0x800000000-0x880000000, ps=0, pe=0x880000000): nr_pdxs=524288 which is
> half the number we need (the correct number is 1048575).
> 
> Taking a line from the x86 code xen/arch/x86/setup.c:setup_max_pdx
> (Julien's suggestion):
> 
>   max_pdx = pfn_to_pdx(top_page - 1) + 1;
> 
> I changed nr_pdxs to
>   
>   nr_pdxs = pfn_to_pdx((pe >> PAGE_SHIFT) - 1) + 1;
> 
> and it works. I think the change is correct because looking at the
> implementation of pfn_to_pdx it is certainly meant to operate on a pfn
> masking bits on it to compensate for the holes. It is not meant to work
> on a size.
> 
> Jan, does it look correct to you too?

Yes. pfn_to_pdx() and friends may only ever be passed actual
PFNs / PDXes, not something that's one above or one below a
valid range. I share Julien's question though: Was it really mere
luck that things have been working to date?

Jan



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