KVM: arm64: Only reset vCPU-scoped feature ID regs once

The general expecation with feature ID registers is that they're 'reset'
exactly once by KVM for the lifetime of a vCPU/VM, such that any
userspace changes to the CPU features / identity are honored after a
vCPU gets reset (e.g. PSCI_ON).

KVM handles what it calls VM-scoped feature ID registers correctly, but
feature ID registers local to a vCPU (CLIDR_EL1, MPIDR_EL1) get wiped
after every reset. What's especially concerning is that a
potentially-changing MPIDR_EL1 breaks MPIDR compression for indexing
mpidr_data, as the mask of useful bits to build the index could change.

This is absolutely no good. Avoid resetting vCPU feature ID registers
more than once.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240502233529.1958459-4-oliver.upton@linux.dev
Signed-off-by: Marc Zyngier <maz@kernel.org>

1 files changed, 24 insertions, 8 deletions

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index bb09ce4bce45..99a485062a62 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1568,6 +1568,14 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, const struct sys_reg_desc *r
 	return IDREG(vcpu->kvm, reg_to_encoding(r));
 }
 
+static bool is_feature_id_reg(u32 encoding)
+{
+	return (sys_reg_Op0(encoding) == 3 &&
+		(sys_reg_Op1(encoding) < 2 || sys_reg_Op1(encoding) == 3) &&
+		sys_reg_CRn(encoding) == 0 &&
+		sys_reg_CRm(encoding) <= 7);
+}
+
 /*
  * Return true if the register's (Op0, Op1, CRn, CRm, Op2) is
  * (3, 0, 0, crm, op2), where 1<=crm<8, 0<=op2<8, which is the range of ID
@@ -1580,6 +1588,11 @@ static inline bool is_vm_ftr_id_reg(u32 id)
 		sys_reg_CRm(id) < 8);
 }
 
+static inline bool is_vcpu_ftr_id_reg(u32 id)
+{
+	return is_feature_id_reg(id) && !is_vm_ftr_id_reg(id);
+}
+
 static inline bool is_aa32_id_reg(u32 id)
 {
 	return (sys_reg_Op0(id) == 3 && sys_reg_Op1(id) == 0 &&
@@ -3522,6 +3535,15 @@ static void reset_vm_ftr_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc
 	IDREG(kvm, id) = reg->reset(vcpu, reg);
 }
 
+static void reset_vcpu_ftr_id_reg(struct kvm_vcpu *vcpu,
+				  const struct sys_reg_desc *reg)
+{
+	if (kvm_vcpu_initialized(vcpu))
+		return;
+
+	reg->reset(vcpu, reg);
+}
+
 /**
  * kvm_reset_sys_regs - sets system registers to reset value
  * @vcpu: The VCPU pointer
@@ -3542,6 +3564,8 @@ void kvm_reset_sys_regs(struct kvm_vcpu *vcpu)
 
 		if (is_vm_ftr_id_reg(reg_to_encoding(r)))
 			reset_vm_ftr_id_reg(vcpu, r);
+		else if (is_vcpu_ftr_id_reg(reg_to_encoding(r)))
+			reset_vcpu_ftr_id_reg(vcpu, r);
 		else
 			r->reset(vcpu, r);
 	}
@@ -3972,14 +3996,6 @@ int kvm_arm_copy_sys_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
 		sys_reg_CRm(r),					\
 		sys_reg_Op2(r))
 
-static bool is_feature_id_reg(u32 encoding)
-{
-	return (sys_reg_Op0(encoding) == 3 &&
-		(sys_reg_Op1(encoding) < 2 || sys_reg_Op1(encoding) == 3) &&
-		sys_reg_CRn(encoding) == 0 &&
-		sys_reg_CRm(encoding) <= 7);
-}
-
 int kvm_vm_ioctl_get_reg_writable_masks(struct kvm *kvm, struct reg_mask_range *range)
 {
 	const void *zero_page = page_to_virt(ZERO_PAGE(0));