KVM: arm64: Advertise FEAT_ECV when possible

We can advertise support for FEAT_ECV if supported on the HW as
long as we limit it to the basic trap bits, and not advertise
CNTPOFF_EL2 support, even if the host has it (the short story
being that CNTPOFF_EL2 is not virtualisable).

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20250220134907.554085-13-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/nested.c

@@ -848,14 +848,16 @@ u64 limit_nv_id_reg(struct kvm *kvm, u32 reg, u64 val)
 		break;
 
 	case SYS_ID_AA64MMFR0_EL1:
-		/* Hide ECV, ExS, Secure Memory */
-		val &= ~(ID_AA64MMFR0_EL1_ECV		|
-			 ID_AA64MMFR0_EL1_EXS		|
+		/* Hide ExS, Secure Memory */
+		val &= ~(ID_AA64MMFR0_EL1_EXS		|
 			 ID_AA64MMFR0_EL1_TGRAN4_2	|
 			 ID_AA64MMFR0_EL1_TGRAN16_2	|
 			 ID_AA64MMFR0_EL1_TGRAN64_2	|
 			 ID_AA64MMFR0_EL1_SNSMEM);
 
+		/* Hide CNTPOFF if present */
+		val = ID_REG_LIMIT_FIELD_ENUM(val, ID_AA64MMFR0_EL1, ECV, IMP);
+
 		/* Disallow unsupported S2 page sizes */
 		switch (PAGE_SIZE) {
 		case SZ_64K: