Re: [PATCH] arm64/mpam: Support partial-core boot for MPAM

[Zeng Heng]

On 2026/1/29 18:11, Ben Horgan wrote:
> Hi Zeng,
>
> I think I've just managed to whitelist your email address. So, all being
> well I'll get your emails in my inbox.
>
> On 1/7/26 03:13, Zeng Heng wrote:
> > Some MPAM MSCs (like L2 MSC) shares the same power domain with its
> > associated CPUs. Therefore, in scenarios where only partial cores power
> > up, the MSCs belonging to the un-powered cores don't need and should not
> > be accessed, otherwise bus-access fault would occur.
>
> The MPAM driver intentionally to waits until all MSCs have been
> discovered before allowing MPAM to be used so that it can check the
> properties of all the MSC and determine the configuration based on full
> knowledge. Once a CPU affine with each MSC has been enabled then MPAM
> will be enabled and usable.
>
> Suppose we weren't to access all MSCs in an asymmetric configuration.
> E.g. if different L2 had different lengths of cache portion bit maps and
> MPAM was enabled with only the CPUs with the same L2 then the driver
> wouldn't know and we'd end up with a bad configuration which would
> become a problem when the other CPUs are eventually turned on.
>
> Hence, I think we should retain the restriction that MPAM is only
> enabled once all MSC are probed. Is this a particularly onerous
> resctriction for you?

I have no objection to the restriction that "MPAM is only enabled once
all MSC are probed." This constraint ensures the driver has complete
knowledge of all Memory System Components before establishing the
configuration.


However, this patch is specifically designed to address CPU core
isolation scenarios (Such as adding the 'isolcpus=xx' kernel command
line parameter).

The patch allows the MPAM driver to successfully complete the
initialization of online MSCs even when the system is booted with
certain cores isolated or disabled. The patch ensures that MPAM
initialization is decoupled from the requirement that all CPUs must be
online during the probing phase.

CPU core isolation is indeed a common production scenario. This
functionality requires the kernel to enable functionalities in the
presence of faulty cores (which cannot be recovered through cold boot).
This ensures system reliability and availability on multi-core
processors where single-core faults.

Without this patch would prevent MPAM from initialization under CPU core
isolation scenarios. Apologies for not mentioning in the patch: we can
verify the functionality by adding 'maxcpus=1' to the boot parameters.

Please let me know if you have any further questions or concerns.


Best Regards,
Zeng Heng