RISC-V: KVM: Allow some SBI extensions to be disabled by default

Currently, all SBI extensions are enabled by default which is
problematic for SBI extensions (such as DBCN) which are forwarded
to the KVM user-space because we might have an older KVM user-space
which is not aware/ready to handle newer SBI extensions. Ideally,
the SBI extensions forwarded to the KVM user-space must be
disabled by default.

To address above, we allow certain SBI extensions to be disabled
by default so that KVM user-space must explicitly enable such
SBI extensions to receive forwarded calls from Guest VCPU.
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/include/asm/kvm_vcpu_sbi.h

@@ -35,6 +35,9 @@ struct kvm_vcpu_sbi_return {
 struct kvm_vcpu_sbi_extension {
 	unsigned long extid_start;
 	unsigned long extid_end;
+
+	bool default_unavail;
+
 	/**
 	 * SBI extension handler. It can be defined for a given extension or group of
 	 * extension. But it should always return linux error codes rather than SBI
@@ -59,6 +62,7 @@ int kvm_riscv_vcpu_get_reg_sbi_ext(struct kvm_vcpu *vcpu,
 const struct kvm_vcpu_sbi_extension *kvm_vcpu_sbi_find_ext(
 				struct kvm_vcpu *vcpu, unsigned long extid);
 int kvm_riscv_vcpu_sbi_ecall(struct kvm_vcpu *vcpu, struct kvm_run *run);
+void kvm_riscv_vcpu_sbi_init(struct kvm_vcpu *vcpu);
 
 #ifdef CONFIG_RISCV_SBI_V01
 extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_v01;

arch/riscv/kvm/vcpu.c

@@ -141,6 +141,12 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 	if (rc)
 		return rc;
 
+	/*
+	 * Setup SBI extensions
+	 * NOTE: This must be the last thing to be initialized.
+	 */
+	kvm_riscv_vcpu_sbi_init(vcpu);
+
 	/* Reset VCPU */
 	kvm_riscv_reset_vcpu(vcpu);
 

arch/riscv/kvm/vcpu_sbi.c

@@ -155,14 +155,8 @@ static int riscv_vcpu_set_sbi_ext_single(struct kvm_vcpu *vcpu,
 	if (!sext)
 		return -ENOENT;
 
-	/*
-	 * We can't set the extension status to available here, since it may
-	 * have a probe() function which needs to confirm availability first,
-	 * but it may be too early to call that here. We can set the status to
-	 * unavailable, though.
-	 */
-	if (!reg_val)
-		scontext->ext_status[sext->ext_idx] =
+	scontext->ext_status[sext->ext_idx] = (reg_val) ?
+			KVM_RISCV_SBI_EXT_AVAILABLE :
 			KVM_RISCV_SBI_EXT_UNAVAILABLE;
 
 	return 0;
@@ -188,16 +182,8 @@ static int riscv_vcpu_get_sbi_ext_single(struct kvm_vcpu *vcpu,
 	if (!sext)
 		return -ENOENT;
 
-	/*
-	 * If the extension status is still uninitialized, then we should probe
-	 * to determine if it's available, but it may be too early to do that
-	 * here. The best we can do is report that the extension has not been
-	 * disabled, i.e. we return 1 when the extension is available and also
-	 * when it only may be available.
-	 */
-	*reg_val = scontext->ext_status[sext->ext_idx] !=
-				KVM_RISCV_SBI_EXT_UNAVAILABLE;
-
+	*reg_val = scontext->ext_status[sext->ext_idx] ==
+				KVM_RISCV_SBI_EXT_AVAILABLE;
 	return 0;
 }
 
@@ -337,18 +323,8 @@ const struct kvm_vcpu_sbi_extension *kvm_vcpu_sbi_find_ext(
 			    scontext->ext_status[entry->ext_idx] ==
 						KVM_RISCV_SBI_EXT_AVAILABLE)
 				return ext;
-			if (scontext->ext_status[entry->ext_idx] ==
-						KVM_RISCV_SBI_EXT_UNAVAILABLE)
-				return NULL;
-			if (ext->probe && !ext->probe(vcpu)) {
-				scontext->ext_status[entry->ext_idx] =
-					KVM_RISCV_SBI_EXT_UNAVAILABLE;
-				return NULL;
-			}
 
-			scontext->ext_status[entry->ext_idx] =
-				KVM_RISCV_SBI_EXT_AVAILABLE;
-			return ext;
+			return NULL;
 		}
 	}
 
@@ -419,3 +395,26 @@ int kvm_riscv_vcpu_sbi_ecall(struct kvm_vcpu *vcpu, struct kvm_run *run)
 
 	return ret;
 }
+
+void kvm_riscv_vcpu_sbi_init(struct kvm_vcpu *vcpu)
+{
+	struct kvm_vcpu_sbi_context *scontext = &vcpu->arch.sbi_context;
+	const struct kvm_riscv_sbi_extension_entry *entry;
+	const struct kvm_vcpu_sbi_extension *ext;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(sbi_ext); i++) {
+		entry = &sbi_ext[i];
+		ext = entry->ext_ptr;
+
+		if (ext->probe && !ext->probe(vcpu)) {
+			scontext->ext_status[entry->ext_idx] =
+				KVM_RISCV_SBI_EXT_UNAVAILABLE;
+			continue;
+		}
+
+		scontext->ext_status[entry->ext_idx] = ext->default_unavail ?
+					KVM_RISCV_SBI_EXT_UNAVAILABLE :
+					KVM_RISCV_SBI_EXT_AVAILABLE;
+	}
+}