[PATCH v5 02/41] KVM: arm64: Preserve host MPAM configuration when changing traps

Ben Horgan
2026-02-24T17:57:45Z

When kvm enables or disables MPAM traps to EL2 it clears all other bits in
MPAM2_EL2. Notably, it clears the partition ids (PARTIDs) and performance
monitoring groups (PMGs). Avoid changing these bits in anticipation of
adding support for MPAM in the kernel. Otherwise, on a VHE system with the
host running at EL2 where MPAM2_EL2 and MPAM1_EL1 access the same register,
any attempt to use MPAM to monitor or partition resources for kernel space
would be foiled by running a KVM guest. Additionally, MPAM2_EL2.EnMPAMSM is
always set to 0 which causes MPAMSM_EL1 to always trap. Keep EnMPAMSM set
to 1 when not in a guest so that the kernel can use MPAMSM_EL1.

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>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
arch/arm64/kvm/hyp/include/hyp/switch.h | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index 2597e8bda867..0b50ddd530f3 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -267,7 +267,8 @@ static inline void __deactivate_traps_hfgxtr(struct kvm_vcpu *vcpu)

static inline void __activate_traps_mpam(struct kvm_vcpu *vcpu)
{
- u64 r = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1;
+ u64 clr = MPAM2_EL2_EnMPAMSM;
+ u64 set = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1;

if (!system_supports_mpam())
return;
@@ -277,18 +278,21 @@ static inline void __activate_traps_mpam(struct kvm_vcpu *vcpu)
write_sysreg_s(MPAMHCR_EL2_TRAP_MPAMIDR_EL1, SYS_MPAMHCR_EL2);
} else {
/* From v1.1 TIDR can trap MPAMIDR, set it unconditionally */
- r |= MPAM2_EL2_TIDR;
+ set |= MPAM2_EL2_TIDR;
}

- write_sysreg_s(r, SYS_MPAM2_EL2);
+ sysreg_clear_set_s(SYS_MPAM2_EL2, clr, set);
}

static inline void __deactivate_traps_mpam(void)
{
+ u64 clr = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1 | MPAM2_EL2_TIDR;
+ u64 set = MPAM2_EL2_EnMPAMSM;
+
if (!system_supports_mpam())
return;

- write_sysreg_s(0, SYS_MPAM2_EL2);
+ sysreg_clear_set_s(SYS_MPAM2_EL2, clr, set);

if (system_supports_mpam_hcr())
write_sysreg_s(MPAMHCR_HOST_FLAGS, SYS_MPAMHCR_EL2);
--
2.43.0