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

[Bradley Morgan <brads@mainlining.org>]

On 4 September 2026 11:31:31 BST, Fuad Tabba <fuad.tabba@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 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@arm.com>
>Link: https://lore.kernel.org/all/CA+EHjTxeWxZiuSmnKLGLxTBXP4oJT7-LuffbPAyCSZZ5TW=5Ew@mail.gmail.com/
>Signed-off-by: Fuad Tabba <fuad.tabba@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.

Still LGTM, the only arm device I own is pixel 7, and also a modern arm
test-peoples-crap box. So from what I recall, my tag stands 

>    
>    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
>+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
>

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