KVM: arm64: Wire up CP15 feature registers to their AArch64 equivalents

KVM currently does not trap ID register accesses from an AArch32 EL1.
This is painful for a couple of reasons. Certain unimplemented features
are visible to AArch32 EL1, as we limit PMU to version 3 and the debug
architecture to v8.0. Additionally, we attempt to paper over
heterogeneous systems by using register values that are safe
system-wide. All this hard work is completely sidestepped because KVM
does not set TID3 for AArch32 guests.

Fix up handling of CP15 feature registers by simply rerouting to their
AArch64 aliases. Punt setting HCR_EL2.TID3 to a later change, as we need
to fix up the oddball CP10 feature registers still.
Signed-off-by: Oliver Upton <oupton@google.com>
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220503060205.2823727-4-oupton@google.com

arch/arm64/kvm/sys_regs.c

@@ -2344,34 +2344,73 @@ static int kvm_handle_cp_64(struct kvm_vcpu *vcpu,
 	return 1;
 }
 
+static bool emulate_sys_reg(struct kvm_vcpu *vcpu, struct sys_reg_params *params);
+
+/**
+ * kvm_emulate_cp15_id_reg() - Handles an MRC trap on a guest CP15 access where
+ *			       CRn=0, which corresponds to the AArch32 feature
+ *			       registers.
+ * @vcpu: the vCPU pointer
+ * @params: the system register access parameters.
+ *
+ * Our cp15 system register tables do not enumerate the AArch32 feature
+ * registers. Conveniently, our AArch64 table does, and the AArch32 system
+ * register encoding can be trivially remapped into the AArch64 for the feature
+ * registers: Append op0=3, leaving op1, CRn, CRm, and op2 the same.
+ *
+ * According to DDI0487G.b G7.3.1, paragraph "Behavior of VMSAv8-32 32-bit
+ * System registers with (coproc=0b1111, CRn==c0)", read accesses from this
+ * range are either UNKNOWN or RES0. Rerouting remains architectural as we
+ * treat undefined registers in this range as RAZ.
+ */
+static int kvm_emulate_cp15_id_reg(struct kvm_vcpu *vcpu,
+				   struct sys_reg_params *params)
+{
+	int Rt = kvm_vcpu_sys_get_rt(vcpu);
+
+	/* Treat impossible writes to RO registers as UNDEFINED */
+	if (params->is_write) {
+		unhandled_cp_access(vcpu, params);
+		return 1;
+	}
+
+	params->Op0 = 3;
+
+	/*
+	 * All registers where CRm > 3 are known to be UNKNOWN/RAZ from AArch32.
+	 * Avoid conflicting with future expansion of AArch64 feature registers
+	 * and simply treat them as RAZ here.
+	 */
+	if (params->CRm > 3)
+		params->regval = 0;
+	else if (!emulate_sys_reg(vcpu, params))
+		return 1;
+
+	vcpu_set_reg(vcpu, Rt, params->regval);
+	return 1;
+}
+
 /**
  * kvm_handle_cp_32 -- handles a mrc/mcr trap on a guest CP14/CP15 access
  * @vcpu: The VCPU pointer
  * @run:  The kvm_run struct
  */
 static int kvm_handle_cp_32(struct kvm_vcpu *vcpu,
+			    struct sys_reg_params *params,
 			    const struct sys_reg_desc *global,
 			    size_t nr_global)
 {
-	struct sys_reg_params params;
-	u32 esr = kvm_vcpu_get_esr(vcpu);
 	int Rt  = kvm_vcpu_sys_get_rt(vcpu);
 
-	params.CRm = (esr >> 1) & 0xf;
-	params.regval = vcpu_get_reg(vcpu, Rt);
-	params.is_write = ((esr & 1) == 0);
-	params.CRn = (esr >> 10) & 0xf;
-	params.Op0 = 0;
-	params.Op1 = (esr >> 14) & 0x7;
-	params.Op2 = (esr >> 17) & 0x7;
+	params->regval = vcpu_get_reg(vcpu, Rt);
 
-	if (emulate_cp(vcpu, &params, global, nr_global)) {
-		if (!params.is_write)
-			vcpu_set_reg(vcpu, Rt, params.regval);
+	if (emulate_cp(vcpu, params, global, nr_global)) {
+		if (!params->is_write)
+			vcpu_set_reg(vcpu, Rt, params->regval);
 		return 1;
 	}
 
-	unhandled_cp_access(vcpu, &params);
+	unhandled_cp_access(vcpu, params);
 	return 1;
 }
 
@@ -2382,7 +2421,20 @@ int kvm_handle_cp15_64(struct kvm_vcpu *vcpu)
 
 int kvm_handle_cp15_32(struct kvm_vcpu *vcpu)
 {
-	return kvm_handle_cp_32(vcpu, cp15_regs, ARRAY_SIZE(cp15_regs));
+	struct sys_reg_params params;
+
+	params = esr_cp1x_32_to_params(kvm_vcpu_get_esr(vcpu));
+
+	/*
+	 * Certain AArch32 ID registers are handled by rerouting to the AArch64
+	 * system register table. Registers in the ID range where CRm=0 are
+	 * excluded from this scheme as they do not trivially map into AArch64
+	 * system register encodings.
+	 */
+	if (params.Op1 == 0 && params.CRn == 0 && params.CRm)
+		return kvm_emulate_cp15_id_reg(vcpu, &params);
+
+	return kvm_handle_cp_32(vcpu, &params, cp15_regs, ARRAY_SIZE(cp15_regs));
 }
 
 int kvm_handle_cp14_64(struct kvm_vcpu *vcpu)
@@ -2392,7 +2444,11 @@ int kvm_handle_cp14_64(struct kvm_vcpu *vcpu)
 
 int kvm_handle_cp14_32(struct kvm_vcpu *vcpu)
 {
-	return kvm_handle_cp_32(vcpu, cp14_regs, ARRAY_SIZE(cp14_regs));
+	struct sys_reg_params params;
+
+	params = esr_cp1x_32_to_params(kvm_vcpu_get_esr(vcpu));
+
+	return kvm_handle_cp_32(vcpu, &params, cp14_regs, ARRAY_SIZE(cp14_regs));
 }
 
 static bool is_imp_def_sys_reg(struct sys_reg_params *params)

arch/arm64/kvm/sys_regs.h

@@ -35,6 +35,13 @@ struct sys_reg_params {
 				  .Op2 = ((esr) >> 17) & 0x7,                  \
 				  .is_write = !((esr) & 1) })
 
+#define esr_cp1x_32_to_params(esr)						\
+	((struct sys_reg_params){ .Op1 = ((esr) >> 14) & 0x7,			\
+				  .CRn = ((esr) >> 10) & 0xf,			\
+				  .CRm = ((esr) >> 1) & 0xf,			\
+				  .Op2 = ((esr) >> 17) & 0x7,			\
+				  .is_write = !((esr) & 1) })
+
 struct sys_reg_desc {
 	/* Sysreg string for debug */
 	const char *name;