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arm64: mpam: Document when to set arm64.nompam

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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 <ben.horgan@arm.com>
Link: https://lore.kernel.org/all/CA+EHjTxeWxZiuSmnKLGLxTBXP4oJT7-LuffbPAyCSZZ5TW=5Ew@mail.gmail.com/
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Reviewed-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
This commit is contained in:
Fuad Tabba 2026-09-08 15:56:22 +01:00 committed by Catalin Marinas
parent 93f51579e7
commit eb0fde98fe
2 changed files with 28 additions and 1 deletions
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3
Documentation/admin-guide/kernel-parameters.txt
View File

@ -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

26
Documentation/arch/arm64/mpam.rst
View File

@ -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