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

[Julien Grall <julien.grall@linaro.org>]

Hi Ian,

On 09/09/14 05:16, Ian Campbell 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?

I don't have any things to add for now on this patch. The ARM64 part 
looks good to me. I'm waiting to see the ARM32 one.

Regards,

-- 
Julien Grall