Re: [PATCH hyperv-next v6 01/11] arm64: kvm, smccc: Introduce and use API for detecting hypervisor presence

Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx> · Fri, 14 Mar 2025 17:27:59 -0700

On 3/14/2025 5:19 PM, Roman Kisel wrote:
The KVM/arm64 uses SMCCC to detect hypervisor presence. That code is
private, and it follows the SMCCC specification. Other existing and
emerging hypervisor guest implementations can and should use that
standard approach as well.

Factor out a common infrastructure that the guests can use, update KVM
to employ the new API. The central notion of the SMCCC method is the
UUID of the hypervisor, and the API follows that.

No functional changes. Validated with a KVM/arm64 guest.

Signed-off-by: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx>
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>

While the change is Acked, here is the caveat maybe.

This patch produces warnings wtih sparse and CHECK_ENDING.
That said, the kernel build produces a whole lot more other warnings
from building with sparse by itself and/or with CHECK_ENDING.

I am not sure how to proceed with that, thinking I should
not add warnings yet at the same time there are many others.
Not certain if folks take these signals as fyi or blockers.

Decided to send V6 to trim the V5 discussion threads by implementing
suggestions and fixes.

---
  arch/arm64/kvm/hypercalls.c        |  5 +--
  drivers/firmware/smccc/kvm_guest.c | 10 ++----
  drivers/firmware/smccc/smccc.c     | 19 +++++++++++
  include/linux/arm-smccc.h          | 55 +++++++++++++++++++++++++++---
  4 files changed, 73 insertions(+), 16 deletions(-)

diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 27ce4cb44904..92b9bc1ea8e8 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -353,10 +353,7 @@ int kvm_smccc_call_handler(struct kvm_vcpu *vcpu)
  			val[0] = gpa;
  		break;
  	case ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID:
-		val[0] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_0;
-		val[1] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_1;
-		val[2] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_2;
-		val[3] = ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3;
+		UUID_TO_SMCCC_RES(ARM_SMCCC_VENDOR_HYP_UID_KVM, val);
  		break;
  	case ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID:
  		val[0] = smccc_feat->vendor_hyp_bmap;
diff --git a/drivers/firmware/smccc/kvm_guest.c b/drivers/firmware/smccc/kvm_guest.c
index f3319be20b36..b5084b309ea0 100644
--- a/drivers/firmware/smccc/kvm_guest.c
+++ b/drivers/firmware/smccc/kvm_guest.c
@@ -14,17 +14,11 @@ static DECLARE_BITMAP(__kvm_arm_hyp_services, ARM_SMCCC_KVM_NUM_FUNCS) __ro_afte
  
  void __init kvm_init_hyp_services(void)
  {
+	uuid_t kvm_uuid = ARM_SMCCC_VENDOR_HYP_UID_KVM;
  	struct arm_smccc_res res;
  	u32 val[4];
  
-	if (arm_smccc_1_1_get_conduit() != SMCCC_CONDUIT_HVC)
-		return;
-
-	arm_smccc_1_1_invoke(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, &res);
-	if (res.a0 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_0 ||
-	    res.a1 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_1 ||
-	    res.a2 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_2 ||
-	    res.a3 != ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3)
+	if (!arm_smccc_hyp_present(&kvm_uuid))
  		return;
  
  	memset(&res, 0, sizeof(res));
diff --git a/drivers/firmware/smccc/smccc.c b/drivers/firmware/smccc/smccc.c
index a74600d9f2d7..7399f27d58e5 100644
--- a/drivers/firmware/smccc/smccc.c
+++ b/drivers/firmware/smccc/smccc.c
@@ -67,6 +67,25 @@ s32 arm_smccc_get_soc_id_revision(void)
  }
  EXPORT_SYMBOL_GPL(arm_smccc_get_soc_id_revision);
  
+bool arm_smccc_hyp_present(const uuid_t *hyp_uuid)
+{
+	struct arm_smccc_res res = {};
+
+	if (arm_smccc_1_1_get_conduit() != SMCCC_CONDUIT_HVC)
+		return false;
+	arm_smccc_1_1_hvc(ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID, &res);
+	if (res.a0 == SMCCC_RET_NOT_SUPPORTED)
+		return false;
+
+	return ({
+		const uuid_t uuid = SMCCC_RES_TO_UUID(res.a0, res.a1, res.a2, res.a3);
+		const bool present = uuid_equal(&uuid, hyp_uuid);
+
+		present;
+	});
+}
+EXPORT_SYMBOL_GPL(arm_smccc_hyp_present);
+
  static int __init smccc_devices_init(void)
  {
  	struct platform_device *pdev;
diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
index 67f6fdf2e7cd..726f18221f1c 100644
--- a/include/linux/arm-smccc.h
+++ b/include/linux/arm-smccc.h
@@ -7,6 +7,11 @@
  
  #include <linux/args.h>
  #include <linux/init.h>
+
+#ifndef __ASSEMBLER__
+#include <linux/uuid.h>
+#endif
+
  #include <uapi/linux/const.h>
  
  /*
@@ -107,10 +112,10 @@
  			   ARM_SMCCC_FUNC_QUERY_CALL_UID)
  
  /* KVM UID value: 28b46fb6-2ec5-11e9-a9ca-4b564d003a74 */
-#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_0	0xb66fb428U
-#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_1	0xe911c52eU
-#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_2	0x564bcaa9U
-#define ARM_SMCCC_VENDOR_HYP_UID_KVM_REG_3	0x743a004dU
+#define ARM_SMCCC_VENDOR_HYP_UID_KVM UUID_INIT(\
+	0xb66fb428, 0xc52e, 0xe911, \
+	0xa9, 0xca, 0x4b, 0x56, \
+	0x4d, 0x00, 0x3a, 0x74)
  
  /* KVM "vendor specific" services */
  #define ARM_SMCCC_KVM_FUNC_FEATURES		0
@@ -333,6 +338,48 @@ s32 arm_smccc_get_soc_id_version(void);
   */
  s32 arm_smccc_get_soc_id_revision(void);
  
+#ifndef __ASSEMBLER__
+
+/**
+ * arm_smccc_hyp_present(const uuid_t *hyp_uuid)
+ *
+ * Returns `true` if the hypervisor advertises its presence via SMCCC.
+ *
+ * When the function returns `false`, the caller shall not assume that
+ * there is no hypervisor running. Instead, the caller must fall back to
+ * other approaches if any are available.
+ */
+bool arm_smccc_hyp_present(const uuid_t *hyp_uuid);
+
+#define SMCCC_RES_TO_UUID(r0, r1, r2, r3) \
+	UUID_INIT( \
+		cpu_to_le32(lower_32_bits(r0)), \
+		cpu_to_le32(lower_32_bits(r1)) & 0xffff, \
+		cpu_to_le32(lower_32_bits(r1)) >> 16, \
+		cpu_to_le32(lower_32_bits(r2)) & 0xff, \
+		(cpu_to_le32(lower_32_bits(r2)) >> 8) & 0xff, \
+		(cpu_to_le32(lower_32_bits(r2)) >> 16) & 0xff, \
+		(cpu_to_le32(lower_32_bits(r2)) >> 24) & 0xff, \
+		cpu_to_le32(lower_32_bits(r3)) & 0xff, \
+		(cpu_to_le32(lower_32_bits(r3)) >> 8) & 0xff, \
+		(cpu_to_le32(lower_32_bits(r3)) >> 16) & 0xff, \
+		(cpu_to_le32(lower_32_bits(r3)) >> 24) & 0xff \
+	)
+
+#define UUID_TO_SMCCC_RES(uuid_init, regs) do { \
+		const uuid_t uuid = uuid_init; \
+		(regs)[0] = le32_to_cpu((u32)uuid.b[0] | (uuid.b[1] << 8) | \
+						((uuid.b[2]) << 16) | ((uuid.b[3]) << 24)); \
+		(regs)[1] = le32_to_cpu((u32)uuid.b[4] | (uuid.b[5] << 8) | \
+						((uuid.b[6]) << 16) | ((uuid.b[7]) << 24)); \
+		(regs)[2] = le32_to_cpu((u32)uuid.b[8] | (uuid.b[9] << 8) | \
+						((uuid.b[10]) << 16) | ((uuid.b[11]) << 24)); \
+		(regs)[3] = le32_to_cpu((u32)uuid.b[12] | (uuid.b[13] << 8) | \
+						((uuid.b[14]) << 16) | ((uuid.b[15]) << 24)); \
+	} while (0)
+
+#endif /* !__ASSEMBLER__ */
+
  /**
   * struct arm_smccc_res - Result from SMC/HVC call
   * @a0-a3 result values from registers 0 to 3

--
Thank you,
Roman








