linux-kernel - Re: [PATCH v3 13/47] KVM: arm64: Use kernel-space partid configuration for hypercalls

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Message-ID: <b9e10419-b4de-4cb6-93b6-07e878d2072a@arm.com>
Date: Wed, 14 Jan 2026 14:39:58 +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 13/47] KVM: arm64: Use kernel-space partid
 configuration for hypercalls

Hi Marc,

On 1/14/26 12:09, Marc Zyngier wrote:
> On Mon, 12 Jan 2026 16:58:40 +0000,
> Ben Horgan <ben.horgan@....com> wrote:
>>
>> On nVHE systems whether or not MPAM is enabled, EL2 continues to use
>> partid-0 for hypercalls, even when the host may have configured its kernel
>> threads to use a different partid. 0 may have been assigned to another
>> task. Copy the EL1 MPAM register to EL2. This ensures hypercalls use the
>> same partid as the kernel thread does on the host.
>>
>> Signed-off-by: Ben Horgan <ben.horgan@....com>
>> ---
>> Changes since v2:
>> Use mask
>> Use read_sysreg_el1 to cope with hvhe
>> ---
>>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
>> index a7c689152f68..ad99d8a73a9e 100644
>> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
>> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
>> @@ -635,6 +635,14 @@ static void handle_host_hcall(struct kvm_cpu_context *host_ctxt)
>>  	unsigned long hcall_min = 0;
>>  	hcall_t hfn;
>>  
>> +	if (system_supports_mpam()) {
>> +		u64 mask = MPAM1_EL1_PARTID_D | MPAM1_EL1_PARTID_I |
>> +			MPAM1_EL1_PMG_D | MPAM1_EL1_PMG_I;
>> +
>> +		write_sysreg_s(read_sysreg_el1(SYS_MPAM1) & mask, SYS_MPAM2_EL2);
>> +		isb();
>> +	}
> 
> Is it really OK to not preserve the rest of MPAM2_EL2? This explicitly
> clears MPAM2_EL2.MPAMEN, which feels counter-productive.
> 
> 	M.
> 

There are 3 things to consider:
1. traps - these are only relevant when we leave EL2 and are dealt with
in __activate_traps_mpam(). (This also covers EnMPAMSM which is a
not-trap bit.)
2. MPAM2_EL2.MPAMEN - this is read only as long as we have an EL3 and if
we don't have EL3 will be 0 anyway from el2_setup.h and MPAM won't be
considered supported in the kernel.
3. The alternate partid space fields which are kept as zero and relate
to FEAT_RME.

So, safe. Ok with you or would you rather I make it more obviously safe?

Thanks,

Ben