Re: [PATCH v2 05/45] KVM: arm64: Preserve host MPAM configuration when changing traps

Ben Horgan
2026-01-02T11:43:49Z

Hi Oliver,

On 12/19/25 20:01, Oliver Upton wrote:
> Hi Ben,
>
> On Fri, Dec 19, 2025 at 06:11:07PM +0000, Ben Horgan wrote:
>> 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.
>>
>> 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 c5d5e5b86eaf..63195275a8b8 100644
>> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
>> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
>> @@ -269,7 +269,8 @@ 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;
>> @@ -279,18 +280,21 @@ 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);
>
> I'd recommend documenting that writes to MPAM1_EL1 are followed by an
> ISB. Otherwise it isn't obvious here where context synchronization is
> happening (if at all).

Ok, but why are you mentioning it here?

This is just updating the traps while ensuring the partid/pmg are
unchanged. The traps are only relevant once we change exception level
and for activation they are synchronized in __guest_enter() and the eret
when returning to userspace.

>
> Thanks,
> Oliver

Thanks,

Ben