Re: [PATCH v5 4/7] KVM: arm64: Fix missing traps of guest accesses to the MPAM registers

[Oliver Upton <oliver.upton@linux.dev>]

Hi Joey,

On Tue, Oct 15, 2024 at 02:39:20PM +0100, Joey Gouly wrote:
> +static inline void  __activate_traps_mpam(struct kvm_vcpu *vcpu)
> +{
> +	u64 r = MPAM2_EL2_TRAPMPAM0EL1 | MPAM2_EL2_TRAPMPAM1EL1;
> +
> +	if (!cpus_support_mpam())
> +		return;
> +
> +	/* trap guest access to MPAMIDR_EL1 */
> +	if (mpam_cpus_have_mpam_hcr()) {
> +		write_sysreg_s(MPAMHCR_EL2_TRAP_MPAMIDR_EL1, SYS_MPAMHCR_EL2);
> +	} else {
> +		/* From v1.1 TIDR can trap MPAMIDR, set it unconditionally */
> +		r |= MPAM2_EL2_TIDR;
> +	}
> +
> +	write_sysreg_s(r, SYS_MPAM2_EL2);
> +}
> +
> +static inline void __deactivate_traps_mpam(void)
> +{
> +	if (!cpus_support_mpam())
> +		return;
> +
> +	write_sysreg_s(0, SYS_MPAM2_EL2);
> +
> +	if (mpam_cpus_have_mpam_hcr())
> +		write_sysreg_s(MPAMHCR_HOST_FLAGS, SYS_MPAMHCR_EL2);
> +}

TBH, I think our trap configuration should *not* be conditioned on
CONFIG_ARM64_MPAM. Otherwise we're silently allowing the guest to change
things under the nose of KVM/host kernel, assuming an unkind firmware
that left the EL2 trap configuration in a permissive state.

WDYT about detecting the feature && enforcing traps regardless of the Kconfig?

-- 
Thanks,
Oliver