linux-kernel - Re: [PATCH v3 12/47] KVM: arm64: Force guest EL1 to use user-space's partid configuration

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Message-ID: <92041294-b332-4b8b-aeed-d8bbad1a7289@arm.com>
Date: Wed, 14 Jan 2026 14:50:22 +0000
From: Ben Horgan <ben.horgan@....com>
To: Marc Zyngier <maz@...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, oupton@...nel.org, joey.gouly@....com,
 suzuki.poulose@....com, kvmarm@...ts.linux.dev
Subject: Re: [PATCH v3 12/47] KVM: arm64: Force guest EL1 to use user-space's
 partid configuration

Hi Marc,

On 1/14/26 12:06, Marc Zyngier wrote:
> On Mon, 12 Jan 2026 16:58:39 +0000,
> Ben Horgan <ben.horgan@....com> wrote:
>>
>> From: James Morse <james.morse@....com>
>>
>> While we trap the guest's attempts to read/write the MPAM control
>> registers, the hardware continues to use them. Guest-EL0 uses KVM's
>> user-space's configuration, as the value is left in the register, and
>> guest-EL1 uses either the host kernel's configuration, or in the case of
>> VHE, the UNKNOWN reset value of MPAM1_EL1.
>>
>> We want to force the guest-EL1 to use KVM's user-space's MPAM
>> configuration. On nVHE rely on MPAM0_EL1 and MPAM1_EL1 always being
>> programmed the same and on VHE copy MPAM0_EL1 into the guest's
>> MPAM1_EL1. There is no need to restore as this is out of context once TGE
>> is set.
>>
>> Signed-off-by: James Morse <james.morse@....com>
>> Signed-off-by: Ben Horgan <ben.horgan@....com>
>> ---
>> Changes since rfc:
>> Drop the unneeded __mpam_guest_load() in nvhre and the MPAM1_EL1 save restore
>> Defer EL2 handling until next patch
>>
>> Changes since v2:
>> Use mask (Oliver)
>> ---
>>  arch/arm64/kvm/hyp/vhe/sysreg-sr.c | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
>> index f28c6cf4fe1b..9fb8e6628611 100644
>> --- a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
>> +++ b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
>> @@ -183,6 +183,18 @@ void sysreg_restore_guest_state_vhe(struct kvm_cpu_context *ctxt)
>>  }
>>  NOKPROBE_SYMBOL(sysreg_restore_guest_state_vhe);
>>  
>> +/*
>> + * The _EL0 value was written by the host's context switch and belongs to the
>> + * VMM. Copy this into the guest's _EL1 register.
>> + */
>> +static inline void __mpam_guest_load(void)
>> +{
>> +	u64 mask = MPAM0_EL1_PARTID_D | MPAM0_EL1_PARTID_I | MPAM0_EL1_PMG_D | MPAM0_EL1_PMG_I;
>> +
>> +	if (system_supports_mpam())
>> +		write_sysreg_el1(read_sysreg_s(SYS_MPAM0_EL1) & mask, SYS_MPAM1);
>> +}
>> +
>>  /**
>>   * __vcpu_load_switch_sysregs - Load guest system registers to the physical CPU
>>   *
>> @@ -222,6 +234,7 @@ void __vcpu_load_switch_sysregs(struct kvm_vcpu *vcpu)
>>  	 */
>>  	__sysreg32_restore_state(vcpu);
>>  	__sysreg_restore_user_state(guest_ctxt);
>> +	__mpam_guest_load();
> 
> What's the rationale for doing this independently of rest of the MPAM
> stuff in __activate_traps_mpam()?

The __activate_traps_mpam() is relevant even for nvhe but
__mpam_guest_load() is only need in vhe as otherwise we can rely on
MPAM1_EL1 and MPAM0_EL0 having the same partid/pmg configuration
(although this MPAM policy will likely become configurable sometime down
the line). Besides that it just makes the naming less exact.

> 
> 	M.
> 

Thanks,

Ben