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

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Message-ID: <187ac0bf-cd7e-4fed-8236-9097c938ab8c@arm.com>
Date: Fri, 2 Jan 2026 11:43:42 +0000
From: Ben Horgan <ben.horgan@....com>
To: Oliver Upton <oupton@...nel.org>
Cc: amitsinght@...vell.com, baisheng.gao@...soc.com,
 baolin.wang@...ux.alibaba.com, carl@...amperecomputing.com,
 dave.martin@....com, david@...nel.org, dfustini@...libre.com,
 fenghuay@...dia.com, gshan@...hat.com, james.morse@....com,
 jonathan.cameron@...wei.com, kobak@...dia.com, lcherian@...vell.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 peternewman@...gle.com, punit.agrawal@....qualcomm.com,
 quic_jiles@...cinc.com, reinette.chatre@...el.com, rohit.mathew@....com,
 scott@...amperecomputing.com, sdonthineni@...dia.com,
 tan.shaopeng@...itsu.com, xhao@...ux.alibaba.com, catalin.marinas@....com,
 will@...nel.org, corbet@....net, maz@...nel.org, joey.gouly@....com,
 suzuki.poulose@....com, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v2 05/45] KVM: arm64: Preserve host MPAM configuration
 when changing traps

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@....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