KVM: arm64: Make vCPU feature flags consistent VM-wide

To date KVM has allowed userspace to construct asymmetric VMs where
particular features may only be supported on a subset of vCPUs. This
wasn't really the intened usage pattern, and it is a total pain in the
ass to keep working in the kernel. What's more, this is at odds with CPU
features in host userspace, where asymmetric features are largely hidden
or disabled.

It's time to put an end to the whole game. Require all vCPUs in the VM
to have the same feature set, rejecting deviants in the
KVM_ARM_VCPU_INIT ioctl. Preserve some of the vestiges of per-vCPU
feature flags in case we need to reinstate the old behavior for some
limited configurations. Yes, this is a sign of cowardice around a
user-visibile change.

Hoist all of the 32-bit limitations into kvm_vcpu_init_check_features()
to avoid nested attempts to acquire the config_lock, which won't end
well.

Link: https://lore.kernel.org/r/20230609190054.1542113-4-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

2 files changed, 10 insertions, 19 deletions

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index b31b32ecbe2d..b19c35129d1c 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h
@@ -62,12 +62,7 @@ static __always_inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
 #else
 static __always_inline bool vcpu_el1_is_32bit(struct kvm_vcpu *vcpu)
 {
-	struct kvm *kvm = vcpu->kvm;
-
-	WARN_ON_ONCE(!test_bit(KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED,
-			       &kvm->arch.flags));
-
-	return test_bit(KVM_ARCH_FLAG_EL1_32BIT, &kvm->arch.flags);
+	return test_bit(KVM_ARM_VCPU_EL1_32BIT, vcpu->arch.features);
 }
 #endif
 
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 565cad211388..2c8c4ace81ef 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -215,25 +215,21 @@ struct kvm_arch {
 #define KVM_ARCH_FLAG_MTE_ENABLED			1
 	/* At least one vCPU has ran in the VM */
 #define KVM_ARCH_FLAG_HAS_RAN_ONCE			2
-	/*
-	 * The following two bits are used to indicate the guest's EL1
-	 * register width configuration. A value of KVM_ARCH_FLAG_EL1_32BIT
-	 * bit is valid only when KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED is set.
-	 * Otherwise, the guest's EL1 register width has not yet been
-	 * determined yet.
-	 */
-#define KVM_ARCH_FLAG_REG_WIDTH_CONFIGURED		3
-#define KVM_ARCH_FLAG_EL1_32BIT				4
+	/* The vCPU feature set for the VM is configured */
+#define KVM_ARCH_FLAG_VCPU_FEATURES_CONFIGURED		3
 	/* PSCI SYSTEM_SUSPEND enabled for the guest */
-#define KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED		5
+#define KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED		4
 	/* VM counter offset */
-#define KVM_ARCH_FLAG_VM_COUNTER_OFFSET			6
+#define KVM_ARCH_FLAG_VM_COUNTER_OFFSET			5
 	/* Timer PPIs made immutable */
-#define KVM_ARCH_FLAG_TIMER_PPIS_IMMUTABLE		7
+#define KVM_ARCH_FLAG_TIMER_PPIS_IMMUTABLE		6
 	/* SMCCC filter initialized for the VM */
-#define KVM_ARCH_FLAG_SMCCC_FILTER_CONFIGURED		8
+#define KVM_ARCH_FLAG_SMCCC_FILTER_CONFIGURED		7
 	unsigned long flags;
 
+	/* VM-wide vCPU feature set */
+	DECLARE_BITMAP(vcpu_features, KVM_VCPU_MAX_FEATURES);
+
 	/*
 	 * VM-wide PMU filter, implemented as a bitmap and big enough for
 	 * up to 2^10 events (ARMv8.0) or 2^16 events (ARMv8.1+).