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

Marc Zyngier
2026-01-14T12:06:31Z

On Mon, 12 Jan 2026 16:58:39 +0000,
Ben Horgan <ben.horgan@arm.com> wrote:
>
> From: James Morse <james.morse@arm.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@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.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()?

M.

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