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

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Message-ID: <86pl7cl7p7.wl-maz@kernel.org>
Date: Wed, 14 Jan 2026 12:06:28 +0000
From: Marc Zyngier <maz@...nel.org>
To: Ben Horgan <ben.horgan@....com>
Cc: amitsinght@...vell.com,
	baisheng.gao@...soc.com,
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Subject: Re: [PATCH v3 12/47] KVM: arm64: Force guest EL1 to use user-space's partid configuration

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