Re: [PATCH v3 4/4] pdx: Add CONFIG_PDX_COMPRESSION as a common Kconfig option

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Several things.

First, Shawn: PPC has gained a HAS_PDX, the deletion of which needs
merging into this patch.

It was added as part of 4a2f68f909304 which was "minimal to build". 
This series address the issue you presumably encountered where pdx.c
appears to be optional but wasn't.


Do PPC platforms have (or potentially have) sparse RAM banks?

If like x86 the answer is definitely no, then you want to have
PDX_COMPRESSION=n

If the answer is definitely yes always, then you want PDX_COMPRESSION=y

If the answer is system specific, then you want to offer users a choice.


On 08/08/2023 2:02 pm, Alejandro Vallejo wrote:
> diff --git a/xen/common/Kconfig b/xen/common/Kconfig
> index 0d248ab941..2c1d1fc3a2 100644
> --- a/xen/common/Kconfig
> +++ b/xen/common/Kconfig
> @@ -23,6 +23,16 @@ config GRANT_TABLE
>  
>         If unsure, say Y.
>  
> +config PDX_COMPRESSION
> +     bool "PDX (Page inDeX) compression support" if EXPERT

This still needs hiding on x86.  The minimal hack for 4.18 is "if EXPERT
&& !X86".

Other adjustments needed depending on the PPC answer above.

> +     default ARM
> +     help
> +       PDX compression is a technique that allows the hypervisor to
> +       represent physical addresses in a very space-efficient manner.
> +       This is very helpful reducing memory wastage in systems with
> +       memory banks with base addresses far from each other, but carries
> +       a performance cost.

This is still intractable for a non-Xen-maintainer, and inaccurate. 
Whether you get any benefit at all depends on how the sparseness happens
to line up.

---
PDX compression is a technique designed to reduce the memory overhead of
physical memory management on platforms with sparse RAM banks.

If your platform does have sparse RAM banks, enabling PDX compression
may reduce the memory overhead of Xen, but does carry a runtime
performance cost.

If your platform does not have sparse RAM banks, do not enable PDX
compression.
---

~Andrew