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

On Tue, Sep 9, 2014 at 5:16 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Mon, 2014-09-08 at 17:32 -0700, Julien Grall wrote:
>> 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!
> No problem. Do you have an opinion on the patch now?
> Ian.
This patchset would be nice to have merged, as without it half of the
memory on the ARM v8 models (and I think Juno board as well)
is ignored.  I've been running with with patchset for the last month
without problem.


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