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

Ben Horgan
2026-01-14T16:50:57Z

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@arm.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@arm.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