This service allows guests to query the host for frequency of the CPU
that the vCPU is currently running on.

Co-developed-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: David Dai <davidai@google.com>
---
 Documentation/virt/kvm/api.rst                |  8 +++++++
 .../virt/kvm/arm/get_cur_cpufreq.rst          | 21 +++++++++++++++++++
 Documentation/virt/kvm/arm/index.rst          |  1 +
 arch/arm64/include/uapi/asm/kvm.h             |  1 +
 arch/arm64/kvm/arm.c                          |  1 +
 arch/arm64/kvm/hypercalls.c                   | 18 ++++++++++++++++
 include/linux/arm-smccc.h                     |  7 +++++++
 include/uapi/linux/kvm.h                      |  1 +
 tools/arch/arm64/include/uapi/asm/kvm.h       |  1 +
 9 files changed, 59 insertions(+)
 create mode 100644 Documentation/virt/kvm/arm/get_cur_cpufreq.rst

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 62de0768d6aa..b0ff0ad700bf 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8380,6 +8380,14 @@ structure.
 When getting the Modified Change Topology Report value, the attr->addr
 must point to a byte where the value will be stored or retrieved from.
 
+8.40 KVM_CAP_GET_CUR_CPUFREQ
+------------------------
+
+:Architectures: arm64
+
+This capability indicates that KVM supports getting the
+frequency of the current CPU that the vCPU thread is running on.
+
 9. Known KVM API problems
 =========================
 
diff --git a/Documentation/virt/kvm/arm/get_cur_cpufreq.rst b/Documentation/virt/kvm/arm/get_cur_cpufreq.rst
new file mode 100644
index 000000000000..06e0ed5b3868
--- /dev/null
+++ b/Documentation/virt/kvm/arm/get_cur_cpufreq.rst
@@ -0,0 +1,21 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+get_cur_cpufreq support for arm/arm64
+=============================
+
+Get_cur_cpufreq support is used to get current frequency(in KHz) of the
+current CPU that the vCPU thread is running on.
+
+* ARM_SMCCC_VENDOR_HYP_KVM_GET_CUR_CPUFREQ_FUNC_ID: 0x86000040
+
+This hypercall uses the SMC32/HVC32 calling convention:
+
+ARM_SMCCC_VENDOR_HYP_KVM_GET_CUR_CPUFREQ_FUNC_ID
+    ==============    ========    =====================================
+    Function ID:      (uint32)    0x86000040
+    Return Values:    (int32)     NOT_SUPPORTED(-1) on error, or
+                      (uint32)    Frequency in KHz of current CPU that the
+                                  vCPU thread is running on.
+    Endianness:                   Must be the same endianness
+                                  as the host.
+    ==============    ========    =====================================
diff --git a/Documentation/virt/kvm/arm/index.rst b/Documentation/virt/kvm/arm/index.rst
index e84848432158..47afc5c1f24a 100644
--- a/Documentation/virt/kvm/arm/index.rst
+++ b/Documentation/virt/kvm/arm/index.rst
@@ -11,3 +11,4 @@ ARM
    hypercalls
    pvtime
    ptp_kvm
+   get_cur_cpufreq
diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index f8129c624b07..ed8b63e91bdc 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -367,6 +367,7 @@ enum {
 enum {
 	KVM_REG_ARM_VENDOR_HYP_BIT_FUNC_FEAT	= 0,
 	KVM_REG_ARM_VENDOR_HYP_BIT_PTP		= 1,
+	KVM_REG_ARM_VENDOR_HYP_BIT_GET_CUR_CPUFREQ	= 2,
 #ifdef __KERNEL__
 	KVM_REG_ARM_VENDOR_HYP_BMAP_BIT_COUNT,
 #endif
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 3bd732eaf087..f960b136c611 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -220,6 +220,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_VCPU_ATTRIBUTES:
 	case KVM_CAP_PTP_KVM:
 	case KVM_CAP_ARM_SYSTEM_SUSPEND:
+	case KVM_CAP_GET_CUR_CPUFREQ:
 		r = 1;
 		break;
 	case KVM_CAP_SET_GUEST_DEBUG2:
diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 5da884e11337..b3f4b90c024b 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -3,6 +3,9 @@
 
 #include <linux/arm-smccc.h>
 #include <linux/kvm_host.h>
+#include <linux/cpufreq.h>
+#include <linux/sched.h>
+#include <uapi/linux/sched/types.h>
 
 #include <asm/kvm_emulate.h>
 
@@ -16,6 +19,15 @@
 #define KVM_ARM_SMCCC_VENDOR_HYP_FEATURES			\
 	GENMASK(KVM_REG_ARM_VENDOR_HYP_BMAP_BIT_COUNT - 1, 0)
 
+static void kvm_sched_get_cur_cpufreq(struct kvm_vcpu *vcpu, u64 *val)
+{
+	unsigned long ret_freq;
+
+	ret_freq = cpufreq_get(task_cpu(current));
+
+	val[0] = ret_freq;
+}
+
 static void kvm_ptp_get_time(struct kvm_vcpu *vcpu, u64 *val)
 {
 	struct system_time_snapshot systime_snapshot;
@@ -116,6 +128,9 @@ static bool kvm_hvc_call_allowed(struct kvm_vcpu *vcpu, u32 func_id)
 	case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
 		return test_bit(KVM_REG_ARM_VENDOR_HYP_BIT_PTP,
 				&smccc_feat->vendor_hyp_bmap);
+	case ARM_SMCCC_VENDOR_HYP_KVM_GET_CUR_CPUFREQ_FUNC_ID:
+		return test_bit(KVM_REG_ARM_VENDOR_HYP_BIT_GET_CUR_CPUFREQ,
+				&smccc_feat->vendor_hyp_bmap);
 	default:
 		return kvm_hvc_call_default_allowed(func_id);
 	}
@@ -213,6 +228,9 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
 	case ARM_SMCCC_VENDOR_HYP_KVM_PTP_FUNC_ID:
 		kvm_ptp_get_time(vcpu, val);
 		break;
+	case ARM_SMCCC_VENDOR_HYP_KVM_GET_CUR_CPUFREQ_FUNC_ID:
+		kvm_sched_get_cur_cpufreq(vcpu, val);
+		break;
 	case ARM_SMCCC_TRNG_VERSION:
 	case ARM_SMCCC_TRNG_FEATURES:
 	case ARM_SMCCC_TRNG_GET_UUID:
diff --git a/include/linux/arm-smccc.h b/include/linux/arm-smccc.h
index 220c8c60e021..e15f1bdcf3f1 100644
--- a/include/linux/arm-smccc.h
+++ b/include/linux/arm-smccc.h
@@ -112,6 +112,7 @@
 /* KVM "vendor specific" services */
 #define ARM_SMCCC_KVM_FUNC_FEATURES		0
 #define ARM_SMCCC_KVM_FUNC_PTP			1
+#define ARM_SMCCC_KVM_FUNC_GET_CUR_CPUFREQ	64
 #define ARM_SMCCC_KVM_FUNC_FEATURES_2		127
 #define ARM_SMCCC_KVM_NUM_FUNCS			128
 
@@ -138,6 +139,12 @@
 #define KVM_PTP_VIRT_COUNTER			0
 #define KVM_PTP_PHYS_COUNTER			1
 
+#define ARM_SMCCC_VENDOR_HYP_KVM_GET_CUR_CPUFREQ_FUNC_ID		\
+	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,				\
+			   ARM_SMCCC_SMC_32,				\
+			   ARM_SMCCC_OWNER_VENDOR_HYP,			\
+			   ARM_SMCCC_KVM_FUNC_GET_CUR_CPUFREQ)
+
 /* Paravirtualised time calls (defined by ARM DEN0057A) */
 #define ARM_SMCCC_HV_PV_TIME_FEATURES				\
 	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,			\
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index d77aef872a0a..0a1a260243bf 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1184,6 +1184,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
 #define KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP 225
 #define KVM_CAP_PMU_EVENT_MASKED_EVENTS 226
+#define KVM_CAP_GET_CUR_CPUFREQ 512
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/tools/arch/arm64/include/uapi/asm/kvm.h b/tools/arch/arm64/include/uapi/asm/kvm.h
index f8129c624b07..ed8b63e91bdc 100644
--- a/tools/arch/arm64/include/uapi/asm/kvm.h
+++ b/tools/arch/arm64/include/uapi/asm/kvm.h
@@ -367,6 +367,7 @@ enum {
 enum {
 	KVM_REG_ARM_VENDOR_HYP_BIT_FUNC_FEAT	= 0,
 	KVM_REG_ARM_VENDOR_HYP_BIT_PTP		= 1,
+	KVM_REG_ARM_VENDOR_HYP_BIT_GET_CUR_CPUFREQ	= 2,
 #ifdef __KERNEL__
 	KVM_REG_ARM_VENDOR_HYP_BMAP_BIT_COUNT,
 #endif
-- 
2.40.0.348.gf938b09366-goog

On Thursday 30 Mar 2023 at 15:43:37 (-0700), David Dai wrote:
> This service allows guests to query the host for frequency of the CPU
> that the vCPU is currently running on.

I assume the intention here is to achieve scale invariance in the guest
to ensure its PELT signals represent how much work is actually being
done. If so, it's likely the usage of activity monitors will be superior
for this type of thing as that may allow us to drop the baked-in
assumption about vCPU pinning. IIRC, AMUs v2 (arm64-specific obv) have
extended support for virtualization, so I'd suggest looking into
supporting that first.

And assuming we also want to support this on hardware that don't have
AMUs, or don't have the right virt extensions, then the only thing I can
think of is to have the VMM expose non-architectural AMUs to the guest,
maybe emulated using PMUs. If the guest uses Linux, it'll need to grow
support for non-architectural AMUs which is its own can of worms though.

Thanks,
Quentin