[PATCH v5 12/41] KVM: arm64: Use kernel-space partid configuration for hypercalls

[Marc Zyngier]

On Tue, 24 Feb 2026 17:56:51 +0000,
Ben Horgan <ben.horgan at 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.
> 
> Tested-by: Gavin Shan <gshan at redhat.com>
> Tested-by: Shaopeng Tan <tan.shaopeng at jp.fujitsu.com>
> Tested-by: Peter Newman <peternewman at google.com>
> Tested-by: Zeng Heng <zengheng4 at huawei.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng at jp.fujitsu.com>
> Reviewed-by: Jonathan Cameron <jonathan.cameron at huawei.com>
> Signed-off-by: Ben Horgan <ben.horgan at arm.com>
> ---
> Changes since v2:
> Use mask
> Use read_sysreg_el1 to cope with hvhe
> 
> Changes since v3:
> Set MPAM2_EL2.MPAMEN to 1 as we rely on that before and after
> ---
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index e7790097db93..80e71eeddc03 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -638,6 +638,15 @@ 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;
> +		u64 val = MPAM2_EL2_MPAMEN | (read_sysreg_el1(SYS_MPAM1) & mask);
> +
> +		write_sysreg_s(val, SYS_MPAM2_EL2);
> +		isb();
> +	}
> +
>  	/*
>  	 * If pKVM has been initialised then reject any calls to the
>  	 * early "privileged" hypercalls. Note that we cannot reject

It is extremely debatable whether this is desirable:

- pKVM really shouldn't be influenced by what the host does, which
  means reserving PARTIDs and indirecting what the host sees. This can
  be deferred until pKVM is actually useful upstream.

- repeatedly hammering that register plus an ISB on the hot path of a
  hypercall is a sure way to make things worse than they should be,
  and that should be fixed now.

Do you really expect the EL1 settings to change on a regular basis? If
so, I'd rather you use a specific host hypercall, or even a trap to
propagate the EL1 configuration. If not, just set it as part of the
KVM init and be done with it.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.