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

[Marc Zyngier]

On Tue, 24 Feb 2026 17:56:41 +0000,
Ben Horgan <ben.horgan@xxxxxxx> 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@xxxxxxxxxx>
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxxxxx>
> Tested-by: Peter Newman <peternewman@xxxxxxxxxx>
> Tested-by: Zeng Heng <zengheng4@xxxxxxxxxx>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxxxxx>
> Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>
> Reviewed-by: Gavin Shan <gshan@xxxxxxxxxx>
> Signed-off-by: Ben Horgan <ben.horgan@xxxxxxx>
> ---
>  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@xxxxxxxxxx>

	M.

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