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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-09-09 at 13:59 -0700, Julien Grall wrote:
> 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.

Damn, I new I'd forgotten to do something!

I'll take a look at arm32 asap....

Ian.



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