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

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Message-ID: <fe013731-4bbf-47c2-85f8-842f781ae2f3@arm.com>
Date: Wed, 14 Jan 2026 16:50:50 +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 14:39, Ben Horgan wrote:
> 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?

As discussed offline, to avoid having to reason about MPAM2_EL2.MPAMEN
I'll set this bit to 1 in this write as we are already assuming mpam is
enabled and we want to keep it enabled.

> 
> Thanks,
> 
> Ben
> 
> 

Thanks,

Ben