RISC-V: KVM: Probe for SBI extension status

Rather than defaulting the status to available and allowing the user
to set availability, default to uninitialized and only allow the user
to set the status to unavailable. Then, when an extension is first
used, ensure it is available by invoking its probe function, if it
has one (an extension is assumed available if it doesn't have a probe
function). Checking the status in kvm_vcpu_sbi_find_ext() ensures
extension functions cannot be invoked when they're unavailable.
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Anup Patel <anup@brainfault.org>

arch/riscv/include/asm/kvm_vcpu_sbi.h

@@ -15,6 +15,7 @@
 #define KVM_SBI_VERSION_MINOR 0
 
 enum kvm_riscv_sbi_ext_status {
+	KVM_RISCV_SBI_EXT_UNINITIALIZED,
 	KVM_RISCV_SBI_EXT_AVAILABLE,
 	KVM_RISCV_SBI_EXT_UNAVAILABLE,
 };

arch/riscv/kvm/vcpu_sbi.c

@@ -155,8 +155,15 @@ static int riscv_vcpu_set_sbi_ext_single(struct kvm_vcpu *vcpu,
 	if (!sext)
 		return -ENOENT;
 
-	scontext->ext_status[sext->ext_idx] = reg_val ?
-		KVM_RISCV_SBI_EXT_AVAILABLE : KVM_RISCV_SBI_EXT_UNAVAILABLE;
+	/*
+	 * 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] =
+			KVM_RISCV_SBI_EXT_UNAVAILABLE;
 
 	return 0;
 }
@@ -181,8 +188,15 @@ static int riscv_vcpu_get_sbi_ext_single(struct kvm_vcpu *vcpu,
 	if (!sext)
 		return -ENOENT;
 
-	*reg_val = scontext->ext_status[sext->ext_idx] ==
-				KVM_RISCV_SBI_EXT_AVAILABLE;
+	/*
+	 * 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;
 
 	return 0;
 }
@@ -309,19 +323,32 @@ 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 i;
-	const struct kvm_riscv_sbi_extension_entry *sext;
 	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++) {
-		sext = &sbi_ext[i];
-		if (sext->ext_ptr->extid_start <= extid &&
-		    sext->ext_ptr->extid_end >= extid) {
-			if (sext->ext_idx < KVM_RISCV_SBI_EXT_MAX &&
-			    scontext->ext_status[sext->ext_idx] ==
+		entry = &sbi_ext[i];
+		ext = entry->ext_ptr;
+
+		if (ext->extid_start <= extid && ext->extid_end >= extid) {
+			if (entry->ext_idx >= KVM_RISCV_SBI_EXT_MAX ||
+			    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;
-			return sbi_ext[i].ext_ptr;
+			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;
 		}
 	}