[PATCH v5 05/41] arm64: mpam: Re-initialise MPAM regs when CPU comes online

Ben Horgan
2026-02-24T17:58:00Z

From: James Morse <james.morse@arm.com>

Now that the MPAM system registers are expected to have values that change,
reprogram them based on the previous value when a CPU is brought online.

Previously MPAM's 'default PARTID' of 0 was always used for MPAM in
kernel-space as this is the PARTID that hardware guarantees to
reset. Because there are a limited number of PARTID, this value is exposed
to user-space, meaning resctrl changes to the resctrl default group would
also affect kernel threads. Instead, use the task's PARTID value for
kernel work on behalf of user-space too. The default of 0 is kept for both
user-space and kernel-space when MPAM is not enabled.

Tested-by: Gavin Shan <gshan@redhat.com>
Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Tested-by: Peter Newman <peternewman@google.com>
Tested-by: Zeng Heng <zengheng4@huawei.com>
Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since rfc:
CONFIG_MPAM -> CONFIG_ARM64_MPAM
Check mpam_enabled
Comment about relying on ERET for synchronisation
Update commit message

Changes since v3:
Always set MPAM1_EL1.MPAMEN rather than relying on it being read only
---
arch/arm64/kernel/cpufeature.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index c31f8e17732a..c3f900f81653 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -86,6 +86,7 @@
#include <asm/kvm_host.h>
#include <asm/mmu.h>
#include <asm/mmu_context.h>
+#include <asm/mpam.h>
#include <asm/mte.h>
#include <asm/hypervisor.h>
#include <asm/processor.h>
@@ -2492,13 +2493,17 @@ test_has_mpam(const struct arm64_cpu_capabilities *entry, int scope)
static void
cpu_enable_mpam(const struct arm64_cpu_capabilities *entry)
{
- /*
- * Access by the kernel (at EL1) should use the reserved PARTID
- * which is configured unrestricted. This avoids priority-inversion
- * where latency sensitive tasks have to wait for a task that has
- * been throttled to release the lock.
- */
- write_sysreg_s(0, SYS_MPAM1_EL1);
+ int cpu = smp_processor_id();
+ u64 regval = 0;
+
+ if (IS_ENABLED(CONFIG_ARM64_MPAM) && static_branch_likely(&mpam_enabled))
+ regval = READ_ONCE(per_cpu(arm64_mpam_current, cpu));
+
+ write_sysreg_s(regval | MPAM1_EL1_MPAMEN, SYS_MPAM1_EL1);
+ isb();
+
+ /* Synchronising the EL0 write is left until the ERET to EL0 */
+ write_sysreg_s(regval, SYS_MPAM0_EL1);
}

static bool
--
2.43.0