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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 29 Apr 2019, Jan Beulich wrote:
> >>> On 29.04.19 at 17:54, <julien.grall@xxxxxxx> wrote:
> > Anyway, I also tested the change suggested by Stefano. This will 
> > substantially 
> > increase the size of the frametable on platform where the RAM does not 
> > start 
> > at 0.
> > 
> > For instance, on Foundation Model the RAM starts at 2GB. As we don't 
> > compress 
> > any of the first 31 bits, the frametable will now be 28MB bigger than we 
> > currently have (112MB up from 84MB).
> > 
> > So I think what we want is:
> > 
> > nr_pdxs = pfn_to_pdx(end - 1) - pfn_to_pdx(start) + 1;
> > frame_table_base_pdx = pfn_to_pdx(start);
> 
> Yes, if there's only a single memory range at 2Gb, then subtracting
> the base address will of course yield better results. But if there are
> multiple regions, things really depend on the placement of all of
> them.

If we do not compress any RAM addresses below 4G, then we cannot
compensate for any holes in that range. However, at least we can safely
skip the first [0-start] like Julien suggested in the calculation of
nr_pdxs.

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