Re: [PATCH v3] arm64: mpam: Document when to set arm64.nompam

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Sep 28, 2026 at 07:56:14AM +0100, Fuad Tabba wrote:
> On Tue, 8 Sept 2026 at 15:56, 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, 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.
> 
> Gentle ping on this one; it has Ben's and Bradley's R-b. I think it
> still stands regardless of the other cpufeature work going on now.
> Anything else needed for it to go in?

Normally I expect a pull request from Ben or James but they rarely send
one in time ;). I'll apply it directly.

-- 
Catalin