linux-kernel - [PATCH v8 25/43] arm64: Don't expose stolen time for realm guests

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Message-ID: <20250416134208.383984-26-steven.price@arm.com>
Date: Wed, 16 Apr 2025 14:41:47 +0100
From: Steven Price <steven.price@....com>
To: kvm@...r.kernel.org,
	kvmarm@...ts.linux.dev
Cc: Steven Price <steven.price@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>,
	Will Deacon <will@...nel.org>,
	James Morse <james.morse@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	Joey Gouly <joey.gouly@....com>,
	Alexandru Elisei <alexandru.elisei@....com>,
	Christoffer Dall <christoffer.dall@....com>,
	Fuad Tabba <tabba@...gle.com>,
	linux-coco@...ts.linux.dev,
	Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
	Gavin Shan <gshan@...hat.com>,
	Shanker Donthineni <sdonthineni@...dia.com>,
	Alper Gun <alpergun@...gle.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>
Subject: [PATCH v8 25/43] arm64: Don't expose stolen time for realm guests

It doesn't make much sense as a realm guest wouldn't want to trust the
host. It will also need some extra work to ensure that KVM will only
attempt to write into a shared memory region. So for now just disable
it.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
Reviewed-by: Gavin Shan <gshan@...hat.com>
Signed-off-by: Steven Price <steven.price@....com>
---
Changes since v7:
 * Update the documentation to add a note about stolen time being
   unavailable in a realm.
---
 Documentation/virt/kvm/api.rst | 3 +++
 arch/arm64/kvm/arm.c           | 5 ++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 54ae0b98030e..f91b35128748 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8684,6 +8684,9 @@ is supported, than the other should as well and vice versa.  For arm64
 see Documentation/virt/kvm/devices/vcpu.rst "KVM_ARM_VCPU_PVTIME_CTRL".
 For x86 see Documentation/virt/kvm/x86/msr.rst "MSR_KVM_STEAL_TIME".
 
+Note that steal time accounting is not available when a guest is running
+within a Arm CCA realm (machine type KVM_VM_TYPE_ARM_REALM).
+
 8.25 KVM_CAP_S390_DIAG318
 -------------------------
 
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index cacdc10d2a6b..97c5efbe47a6 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -396,7 +396,10 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = system_supports_mte();
 		break;
 	case KVM_CAP_STEAL_TIME:
-		r = kvm_arm_pvtime_supported();
+		if (kvm_is_realm(kvm))
+			r = 0;
+		else
+			r = kvm_arm_pvtime_supported();
 		break;
 	case KVM_CAP_ARM_EL1_32BIT:
 		r = cpus_have_final_cap(ARM64_HAS_32BIT_EL1);
-- 
2.43.0