RE: [PATCH v2 03/17] xen/arm: implement node distance helpers for Arm

[Wei Chen <Wei.Chen@xxxxxxx>]

Hi Jan,

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 2023年1月12日 16:11
> To: Wei Chen <Wei.Chen@xxxxxxx>
> Cc: nd <nd@xxxxxxx>; Stefano Stabellini <sstabellini@xxxxxxxxxx>; Julien
> Grall <julien@xxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>;
> Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; George Dunlap <george.dunlap@xxxxxxxxxx>; Wei
> Liu <wl@xxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2 03/17] xen/arm: implement node distance helpers for
> Arm
> 
> On 12.01.2023 07:31, Wei Chen wrote:
> >> -----Original Message-----
> >> From: Jan Beulich <jbeulich@xxxxxxxx>
> >> Sent: 2023年1月11日 0:47
> >>
> >> On 10.01.2023 09:49, Wei Chen wrote:
> >>> --- a/xen/arch/arm/include/asm/numa.h
> >>> +++ b/xen/arch/arm/include/asm/numa.h
> >>> @@ -28,6 +28,20 @@ enum dt_numa_status {
> >>>      DT_NUMA_OFF,
> >>>  };
> >>>
> >>> +/*
> >>> + * In ACPI spec, 0-9 are the reserved values for node distance,
> >>> + * 10 indicates local node distance, 20 indicates remote node
> >>> + * distance. Set node distance map in device tree will follow
> >>> + * the ACPI's definition.
> >>> + */
> >>> +#define NUMA_DISTANCE_UDF_MIN   0
> >>> +#define NUMA_DISTANCE_UDF_MAX   9
> >>> +#define NUMA_LOCAL_DISTANCE     10
> >>> +#define NUMA_REMOTE_DISTANCE    20
> >>
> >> In the absence of a caller of numa_set_distance() it is entirely
> unclear
> >> whether this tying to ACPI used values is actually appropriate.
> >>
> >
> > From Kernel's NUMA device tree binding, it seems DT NUMA are reusing
> > ACPI used values for distances [1].
> 
> I can't find any mention of ACPI in that doc, so the example values used
> there matching ACPI's may also be coincidental. In no event can a Linux
> kernel doc serve as DT specification. If values are to match ACPI's, I
> expect a DT spec to actually say so.
> 

Unfortunately, the latest device tree spec doesn't have any NUMA
description [1]. But if we define different values for DT NUMA in Xen,
we may have an incompatible with Linux DT.

[1] 
https://github.com/devicetree-org/devicetree-specification/releases/download/v0.4-rc1/devicetree-specification-v0.4-rc1.pdf

Cheers,
Wei Chen

> Jan