[PATCH] arm64: mpam: Document when to set arm64.nompam

[Bradley Morgan]

On 3 September 2026 09:48:09 BST, Fuad Tabba <fuad.tabba at linux.dev> wrote:
>arm64.nompam skips the MPAM2_EL2 and MPAMHCR_EL2 writes in
>finalise_el2_state and leaves the ARM64_MPAM cpucap unset. Where EL3
>firmware has enabled MPAM, the EL2 trap controls are left unwritten, at
>reset values that are UNKNOWN, and KVM still hides MPAM from the guest
>but no longer enables the traps that stop a guest from using it. The
>kernel cannot restore the writes: they trap to EL3 on the firmware the
>option exists for, and MPAM3_EL3.TRAPLOWER cannot be read below EL3.
>
>Say so in mpam.rst, and add the rule and a pointer to the
>kernel-parameters entry.
>
>No functional change intended.
>
>Suggested-by: Ben Horgan <ben.horgan at arm.com>
>Link: https://lore.kernel.org/all/CA+EHjTxeWxZiuSmnKLGLxTBXP4oJT7-LuffbPAyCSZZ5TW=5Ew@mail.gmail.com/

LGTM, cheers 

Reviewed-by: Bradley Morgan <brads at mainlining.org>

>Signed-off-by: Fuad Tabba <fuad.tabba at linux.dev>
>---
> Documentation/admin-guide/kernel-parameters.txt |  3 ++-
> Documentation/arch/arm64/mpam.rst               | 17 +++++++++++++++++
> 2 files changed, 19 insertions(+), 1 deletion(-)
>
>diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>index 68647ff4bdd24..e6d543de3cde5 100644
>--- a/Documentation/admin-guide/kernel-parameters.txt
>+++ b/Documentation/admin-guide/kernel-parameters.txt
>@@ -575,7 +575,8 @@ Kernel parameters
> 			Set instructions support
> 
> 	arm64.nompam	[ARM64] Unconditionally disable Memory Partitioning And
>-			Monitoring support
>+			Monitoring support. Only for a machine that does not
>+			boot without it. See Documentation/arch/arm64/mpam.rst
> 
> 	arm64.nomte	[ARM64] Unconditionally disable Memory Tagging Extension
> 			support
>diff --git a/Documentation/arch/arm64/mpam.rst b/Documentation/arch/arm64/mpam.rst
>index 67fe515ed501c..67c8a95391171 100644
>--- a/Documentation/arch/arm64/mpam.rst
>+++ b/Documentation/arch/arm64/mpam.rst
>@@ -87,6 +87,23 @@ The supported features are:
>   MBWU monitors can be exposed to the user after support for more monitoring
>   scopes is added to resctrl.
> 
>+arm64.nompam
>+============
>+Firmware must enable MPAM at the highest implemented exception level and
>+leave the MPAM system registers accessible to the kernel, or trap the
>+accesses and emulate MPAM as disabled. Where it does neither, the CPUs
>+still advertise MPAM in the ID registers, the first kernel access to an
>+MPAM register traps to EL3, and the boot fails. ``arm64.nompam`` exists
>+for that firmware: it makes the kernel treat the CPUs as not implementing
>+MPAM, so no MPAM system register is accessed. Set it only on a machine
>+that does not boot without it.
>+
>+It is not a way to turn MPAM off. On a system whose firmware has enabled
>+MPAM at EL3, the option leaves the trap controls in MPAM2_EL2 and
>+MPAMHCR_EL2 unwritten, and their reset values are UNKNOWN. KVM still
>+hides MPAM from guests but no longer enables the traps that stop a guest
>+from using it, so a guest may be able to choose its own PARTID and PMG.
>+
> Reporting Bugs
> ==============
> If you are not seeing the counters or controls you expect please share
> the
>


--- Thanks!
https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/