KVM: arm64: Always start with clearing SME flag on load

On each vcpu load, we set the KVM_ARM64_HOST_SME_ENABLED
flag if SME is enabled for EL0 on the host. This is used to
restore the correct state on vpcu put.

However, it appears that nothing ever clears this flag. Once
set, it will stick until the vcpu is destroyed, which has the
potential to spuriously enable SME for userspace. As it turns
out, this is due to the SME code being more or less copied from
SVE, and inheriting the same shortcomings.

We never saw the issue because nothing uses SME, and the amount
of testing is probably still pretty low.

Fixes: 861262ab ("KVM: arm64: Handle SME host state when running guests")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviwed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220528113829.1043361-3-maz@kernel.org

arch/arm64/kvm/fpsimd.c

@@ -94,6 +94,7 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 	 * operations. Do this for ZA as well for now for simplicity.
 	 */
 	if (system_supports_sme()) {
+		vcpu->arch.flags &= ~KVM_ARM64_HOST_SME_ENABLED;
 		if (read_sysreg(cpacr_el1) & CPACR_EL1_SMEN_EL0EN)
 			vcpu->arch.flags |= KVM_ARM64_HOST_SME_ENABLED;