From eb0fde98fee407118bfcb10e38400494e0699201 Mon Sep 17 00:00:00 2001 From: Fuad Tabba Date: Tue, 8 Sep 2026 15:56:22 +0100 Subject: [PATCH] arm64: mpam: Document when to set arm64.nompam 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, MPAM1_EL1 and MPAMSM_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 Link: https://lore.kernel.org/all/CA+EHjTxeWxZiuSmnKLGLxTBXP4oJT7-LuffbPAyCSZZ5TW=5Ew@mail.gmail.com/ Reviewed-by: Bradley Morgan Signed-off-by: Fuad Tabba Reviewed-by: Ben Horgan Signed-off-by: Catalin Marinas --- .../admin-guide/kernel-parameters.txt | 3 ++- Documentation/arch/arm64/mpam.rst | 26 +++++++++++++++++++ 2 files changed, 28 insertions(+), 1 deletion(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 33cd30996e47..fdd671c70575 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 67fe515ed501..e445f4078ee1 100644 --- a/Documentation/arch/arm64/mpam.rst +++ b/Documentation/arch/arm64/mpam.rst @@ -87,6 +87,32 @@ 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, MPAM1_EL1 and +MPAMSM_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