[PATCH v4 2/3] arm_mpam: resctrl: Only enable CDP on a resource that supports it

[Jinjie Ruan]

resctrl_arch_set_cdp_enabled() enables CDP for any cache level that is
requested, even one that has no MPAM cache controls. Mounting resctrl
with -o cdpl2 when only L3 can allocate still turns CDP emulation on for
the whole system, so every CLOSID takes two PARTIDs while resctrl keeps
its CLOSID count at the number of PARTIDs. Control groups in the upper
half then map to PARTIDs beyond mpam_partid_max, which index past the
configuration and monitor arrays and program out of range PARTIDs into
the MSCs. On QEMU with 5 PARTIDs the first monitor read after creating
2 control groups raises MSMONCFG_ID_RANGE for PARTID 5, and MPAM is
disabled.

Do not enable CDP when a resource cannot allocate or is not CDP capable,
as x86 does, so the mount fails instead. Only the enable path is checked,
since unmount disables CDP on both cache levels to restore the MBA control.

A system where only MBA can allocate, or that only has monitors, now
fails a mount with CDP as well: CDP hides the MBA control because
mbw_max is a per-PARTID maximum, leaving nothing for CDP to apply to.

Fixes: 6789fb99282c ("arm_mpam: resctrl: Add CDP emulation")
Link: https://sashiko.dev/#/bug/linux-05c19551-0bba-4a06-aa2c-3c90fab5853d
Suggested-by: Ben Horgan <ben.horgan@xxxxxxx>
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 7.1+
---
 drivers/resctrl/mpam_resctrl.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/resctrl/mpam_resctrl.c b/drivers/resctrl/mpam_resctrl.c
index 573617ba09df..afa3c084c475 100644
--- a/drivers/resctrl/mpam_resctrl.c
+++ b/drivers/resctrl/mpam_resctrl.c
@@ -222,6 +222,10 @@ int resctrl_arch_set_cdp_enabled(enum resctrl_res_level rid, bool enable)
 	if (enable && mpam_partid_max < 1)
 		return -EINVAL;
 
+	if (enable && (!mpam_resctrl_controls[rid].resctrl_res.alloc_capable ||
+		       !mpam_resctrl_controls[rid].resctrl_res.cdp_capable))
+		return -EINVAL;
+
 	/*
 	 * resctrl_arch_set_cdp_enabled() is only called with enable set to
 	 * false on error and unmount.
-- 
2.34.1