Re: [PATCH v2 13/45] KVM: arm64: Force guest EL1 to use user-space's partid configuration

[Oliver Upton <oupton@kernel.org>]

On Fri, Dec 19, 2025 at 06:11:15PM +0000, Ben Horgan 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
> ---
>  arch/arm64/kvm/hyp/vhe/sysreg-sr.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
> index f28c6cf4fe1b..a2e8f6187675 100644
> --- a/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
> +++ b/arch/arm64/kvm/hyp/vhe/sysreg-sr.c
> @@ -183,6 +183,16 @@ 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)
> +{
> +	if (system_supports_mpam())
> +		write_sysreg_el1(read_sysreg_s(SYS_MPAM0_EL1), SYS_MPAM1);

nit: I'd prefer using a mask to extract the fields of interest, guarding
against future architecture doing something silly with the RES0 fields
of these registers.

Thanks,
Oliver