[RFC PATCH 16/16] KVM: arm64/sve: Report and enable SVE API extensions for userspace

[Dave.Martin@arm.com (Dave Martin)]

This patch reports the availability of KVM SVE support to userspace
via a new vcpu feature flag KVM_ARM_VCPU_SVE.  This flag is
reported via the KVM_ARM_PREFERRED_TARGET ioctl.

Userspace can enable the feature by setting the flag for
KVM_ARM_VCPU_INIT.  Without this flag set, SVE-related ioctls and
register access extensions are hidden, and SVE remains disabled
unconditionally for the guest.  This ensures that non-SVE-aware KVM
userspace does not receive a vcpu that it does not understand how
to snapshot or restore correctly.

Storage is allocated for the SVE register state at vcpu init time,
sufficient for the maximum vector length to be exposed to the vcpu.
No attempt is made to allocate the storage lazily for now.  Also,
no attempt is made to resize the storage dynamically, since the
effective vector length of the vcpu can change at each EL0/EL1
transition.  The storage is freed at the vcpu uninit hook.

No particular attempt is made to prevent userspace from creating a
mix of vcpus some of which have SVE enabled and some of which have
it disabled.  This may or may not be useful, but it reflects the
underlying architectural behaviour.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/include/asm/kvm_host.h |  6 +++---
 arch/arm64/include/uapi/asm/kvm.h |  1 +
 arch/arm64/kvm/guest.c            | 19 +++++++++++++------
 arch/arm64/kvm/reset.c            | 14 ++++++++++++++
 4 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index d2084ae..d956cf2 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -44,7 +44,7 @@
 
 #define KVM_MAX_VCPUS VGIC_V3_MAX_CPUS
 
-#define KVM_VCPU_MAX_FEATURES 4
+#define KVM_VCPU_MAX_FEATURES 5
 
 #define KVM_REQ_SLEEP \
 	KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
@@ -439,8 +439,8 @@ static inline void kvm_arch_sync_events(struct kvm *kvm) {}
 static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
 static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
 
-static inline int kvm_arm_arch_vcpu_init(struct kvm_vcpu *vcpu) { return 0; }
-static inline void kvm_arm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
+int kvm_arm_arch_vcpu_init(struct kvm_vcpu *vcpu);
+void kvm_arm_arch_vcpu_uninit(struct kvm_vcpu *vcpu);
 
 void kvm_arm_init_debug(void);
 void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index f54a9b0..6acf276 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -101,6 +101,7 @@ struct kvm_regs {
 #define KVM_ARM_VCPU_EL1_32BIT		1 /* CPU running a 32bit VM */
 #define KVM_ARM_VCPU_PSCI_0_2		2 /* CPU uses PSCI v0.2 */
 #define KVM_ARM_VCPU_PMU_V3		3 /* Support guest PMUv3 */
+#define KVM_ARM_VCPU_SVE		4 /* Allow SVE for guest */
 
 struct kvm_vcpu_init {
 	__u32 target;
diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 5152362..fb7f6aa 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -58,6 +58,16 @@ int kvm_arch_vcpu_setup(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
+int kvm_arm_arch_vcpu_init(struct kvm_vcpu *vcpu)
+{
+	return 0;
+}
+
+void kvm_arm_arch_vcpu_uninit(struct kvm_vcpu *vcpu)
+{
+	kfree(vcpu->arch.sve_state);
+}
+
 static u64 core_reg_offset_from_id(u64 id)
 {
 	return id & ~(KVM_REG_ARCH_MASK | KVM_REG_SIZE_MASK | KVM_REG_ARM_CORE);
@@ -600,12 +610,9 @@ int kvm_vcpu_preferred_target(struct kvm_vcpu_init *init)
 
 	memset(init, 0, sizeof(*init));
 
-	/*
-	 * For now, we don't return any features.
-	 * In future, we might use features to return target
-	 * specific features available for the preferred
-	 * target type.
-	 */
+	/* KVM_ARM_VCPU_SVE understood by KVM_VCPU_INIT */
+	init->features[0] = 1 << KVM_ARM_VCPU_SVE;
+
 	init->target = (__u32)target;
 
 	return 0;
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index a74311b..f63a791 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -110,6 +110,20 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
 			cpu_reset = &default_regs_reset;
 		}
 
+		if (system_supports_sve() &&
+		    test_bit(KVM_ARM_VCPU_SVE, vcpu->arch.features)) {
+			vcpu->arch.flags |= KVM_ARM64_GUEST_HAS_SVE;
+
+			vcpu->arch.sve_max_vl = sve_max_virtualisable_vl;
+
+			vcpu->arch.sve_state = kzalloc(
+				SVE_SIG_REGS_SIZE(
+					sve_vq_from_vl(vcpu->arch.sve_max_vl)),
+				GFP_KERNEL);
+			if (!vcpu->arch.sve_state)
+				return -ENOMEM;
+		}
+
 		break;
 	}
 
-- 
2.1.4