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);
 }
 
 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

On Fri, 19 Dec 2025 18:11:07 +0000
Ben Horgan <ben.horgan@arm.com> 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>
Seems to match what I'd expect from the spec, so LGTM.
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>

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).

Thanks,
Oliver

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