Subject: [PATCH v3 3/9] RISC-V: KVM: Allow some SBI extensions to be disabled by default

From: Anup Patel <[email-masked]>
Date: Fri, 20 Oct 2023 12:51:34 +0530

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 <[email-masked]>
---
 arch/riscv/include/asm/kvm_vcpu_sbi.h |  4 ++
 arch/riscv/kvm/vcpu.c                 |  6 +++
 arch/riscv/kvm/vcpu_sbi.c             | 57 +++++++++++++--------------
 3 files changed, 38 insertions(+), 29 deletions(-)

diff --git a/arch/riscv/include/asm/kvm_vcpu_sbi.h b/arch/riscv/include/asm/kvm_vcpu_sbi.h
index 8d6d4dce8a5e..c02bda5559d7 100644
--- a/arch/riscv/include/asm/kvm_vcpu_sbi.h
+++ b/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;
diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index c061a1c5fe98..e087c809073c 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/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);
 
diff --git a/arch/riscv/kvm/vcpu_sbi.c b/arch/riscv/kvm/vcpu_sbi.c
index 9cd97091c723..bda8b0b33343 100644
--- a/arch/riscv/kvm/vcpu_sbi.c
+++ b/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;
+	}
+}
-- 
2.34.1