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

[Fuad Tabba]

Hi Ben,

On Mon, 7 Sept 2026 at 10:44, Ben Horgan <ben.horgan at arm.com> wrote:
>
> 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.

I can respin a V3 with this added.

> 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/

The trapping respin doesn't reach the nompam path: the override skips
the finalise_el2_state writes whether they clear the traps or set
them, so nothing here needs changing.

Cheers,
/fuad

>
> 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
>