[PATCH v2] arm64: mpam: Document when to set arm64.nompam

[Ben Horgan]

Hi Fuad,

On 04/09/2026 11:31, Fuad Tabba 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 cleared MPAM3_EL3.TRAPLOWER, 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. KVM never saves or restores MPAM0_EL1 and MPAM1_EL1, so
> what one guest writes reaches the next guest and the host. Nor can the
> kernel make the EL2 writes unconditionally: they trap to EL3 on the
> firmware the option exists for, and TRAPLOWER cannot be read below EL3.
> 
> Say so in mpam.rst, and add the rule and a pointer to the
> kernel-parameters entry.
> 
> Suggested-by: Ben Horgan <ben.horgan at arm.com>
> Link: https://lore.kernel.org/all/CA+EHjTxeWxZiuSmnKLGLxTBXP4oJT7-LuffbPAyCSZZ5TW=5Ew@mail.gmail.com/
> Signed-off-by: Fuad Tabba <fuad.tabba at linux.dev>
> ---
> 
> Notes:
>     Changes since v1, the first three from Ben Horgan's review:
>      - Put arm64.nompam under a new "Command line parameters" section.
>      - Separate MPAM3_EL3.MPAMEN from TRAPLOWER, which v1 conflated.
>      - Add that KVM does not save or restore MPAM0_EL1 and MPAM1_EL1.
>      - Drop "No functional change intended." from the commit message.
>     
>     Bradley Morgan's Reviewed-by on v1 is not carried over: both
>     paragraphs, the section heading and the commit message changed.
>     
>     v1: https://lore.kernel.org/all/20260903084809.2027326-1-fuad.tabba@linux.dev/
> 
>  .../admin-guide/kernel-parameters.txt         |  3 ++-
>  Documentation/arch/arm64/mpam.rst             | 25 +++++++++++++++++++
>  2 files changed, 27 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..e66b1359d1282 100644
> --- a/Documentation/arch/arm64/mpam.rst
> +++ b/Documentation/arch/arm64/mpam.rst
> @@ -87,6 +87,31 @@ The supported features are:
>    MBWU monitors can be exposed to the user after support for more monitoring
>    scopes is added to resctrl.
>  
> +Command line parameters
> +=======================
> +
> +arm64.nompam
> +------------
> +Firmware controls MPAM through two bits of MPAM3_EL3. MPAMEN enables
> +it: while set, the PARTID and PMG in the MPAMn_ELx registers label the
> +CPU's memory requests. TRAPLOWER, which resets to 1, traps accesses to
> +the MPAM system registers from the lower exception levels to EL3.
> +Firmware must clear it, or handle the trap and emulate MPAM as disabled.
> +Where it does neither, the CPUs still advertise MPAM in the ID registers,
> +the kernel's MPAM register accesses trap 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. Where firmware has cleared TRAPLOWER,
> +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 read and write MPAM0_EL1 and MPAM1_EL1. KVM

As you are listing registers, MPAMSM_EL1 can go here too.

Generally, I think this documentation change looks good now but holding off on a tag for the moment
in case anything comes up from your MPAM trapping change [1].

[1] https://lore.kernel.org/linux-arm-kernel/20260903160819.831518-1-fuad.tabba@linux.dev/

Thanks,

Ben

> +does not save or restore them, so what one guest writes is still there
> +when the next guest runs on that CPU, and when the host does. With MPAMEN
> +set, EL0 and EL1 requests then carry that PARTID and PMG.
> +
>  Reporting Bugs
>  ==============
>  If you are not seeing the counters or controls you expect please share the