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

Marc Zyngier
2026-03-02T17:52:14Z

On Tue, 24 Feb 2026 17:56:41 +0000,
Ben Horgan <ben.horgan@arm.com> wrote:
>
> When kvm enables or disables MPAM traps to EL2 it clears all other bits in

nit: s/kvm/KVM/g

> 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);

Acked-by: Marc Zyngier <maz@kernel.org>

M.

--
Without deviation from the norm, progress is not possible.