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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.09.14 at 18:40, <Ian.Campbell@xxxxxxxxxx> wrote:
> On Thu, 2014-08-07 at 17:03 +0100, Julien Grall 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.
> 
> I have a feeling I've not explained that so well...

You've done a very good job in fact.

Jan


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