KVM: arm64: Allow pKVM on v1.0 compatible FF-A implementations

pKVM initialization fails on systems with v1.1+ FF-A implementations, as
the hyp does a strict match on the returned version from FFA_VERSION.
This is a stronger assertion than required by the specification, which
requires minor revisions be backwards compatible with earlier revisions
of the same major version.

Relax the check in hyp_ffa_init() to only test the returned major
version. Even though v1.1 broke ABI, the expectation is that firmware
incapable of using the v1.0 ABI return NOT_SUPPORTED instead of a valid
version.
Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230718184537.3220867-1-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/hyp/nvhe/ffa.c

@@ -705,7 +705,20 @@ int hyp_ffa_init(void *pages)
 	if (res.a0 == FFA_RET_NOT_SUPPORTED)
 		return 0;
 
-	if (res.a0 != FFA_VERSION_1_0)
+	/*
+	 * Firmware returns the maximum supported version of the FF-A
+	 * implementation. Check that the returned version is
+	 * backwards-compatible with the hyp according to the rules in DEN0077A
+	 * v1.1 REL0 13.2.1.
+	 *
+	 * Of course, things are never simple when dealing with firmware. v1.1
+	 * broke ABI with v1.0 on several structures, which is itself
+	 * incompatible with the aforementioned versioning scheme. The
+	 * expectation is that v1.x implementations that do not support the v1.0
+	 * ABI return NOT_SUPPORTED rather than a version number, according to
+	 * DEN0077A v1.1 REL0 18.6.4.
+	 */
+	if (FFA_MAJOR_VERSION(res.a0) != 1)
 		return -EOPNOTSUPP;
 
 	arm_smccc_1_1_smc(FFA_ID_GET, 0, 0, 0, 0, 0, 0, 0, &res);