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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2014-09-05 at 09:20 +0100, Jan Beulich wrote:
> >>> 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.

Thanks ;-)


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