Re: [Xen-devel] [PATCH RFC 2/2] xen: arm: Enable physical address space compression (PDX) on arm64

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 04/09/14 09:40, Ian Campbell wrote:
> > I don't really understand what the function is achieving. Could you
> > explain a bit more?
> This is largely derived from an x86 equivalent (see srat_parse_regions,
> they are different because one walks the device tree RAM and the other
> the SRAT), so I hope Jan will correct me if I'm wrong about the
> following:
>
> What it is doing is calculating a mask which corresponds to to the bits
> that are active address bits across the valid memory addresses, i.e. a
> bit which is necessary to unambiguously represent some valid address is
> 1 and a bit which is the same for all addresses is 0.
>
> Using that mask we then find a large run of zeroes from he middle of the
> mask which by construction do not actually get used for addressing. By
> omitting those bits from the PFN we obtain a PDX which is the
> (losslessly) compressed form.
Many thank for the explanation!

--
Julien Grall

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