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

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

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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