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 <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>

0 files changed, 0 insertions, 0 deletions