Re: [PATCH 5/8] arm64: Create cache sysfs directory without ACPI PPTT for hardware prefetch control

[Jeremy Linton]

Hi,

On 2/1/22 05:56, tarumizu.kohei@xxxxxxxxxxx wrote:
> > I am assuming this is ACPI enabled system.
>
> Yes, it is ACPI enabled system.
>
> > This looks bit hacky in my opinion. Before I explore better way of adding it, I would
> > like to check if you have explored ways to add PPTT reading these registers from
> > UEFI/EDK2 as PPTT has other topology information which you will need anyways.
> > That would simplify handling of these cacheinfo sysfs in the kernel. Let me know
> > what are your thoughts ?
>
> The latest firmware of ARM64 machine, FX700 with the A64FX processor
> does not support PPTT.
> I think adding PPTT is the best way to generate cacheinfo sysfs.
> However, it is difficult to modify the firmware to add PPTT, so
> it is not clear when it will be possible.
> Therefore, I would like to implement the function in the kernel on
> the condition that firmware does not support PPTT.

As a bit of a late comment here, I assume you tried injecting the PPTT 
via the initrd (directions in 
admin-guide/acpi/initrd_table_override.txt) then? That is one of the 
usual kernel workarounds for broken/missing ACPI tables.

As mentioned above, besides not providing appropriate topology 
information to userspace, not having the PPTT is also possibly causing 
suboptimal scheduling decisions in the kernel itself.