KVM: arm64: Add debugfs file for guest's ID registers

Debugging ID register setup can be a complicated affair. Give the
kernel hacker a way to dump that state in an easy to parse way.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240214131827.2856277-27-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/include/asm/kvm_host.h

@@ -319,6 +319,9 @@ struct kvm_arch {
 	/* PMCR_EL0.N value for the guest */
 	u8 pmcr_n;
 
+	/* Iterator for idreg debugfs */
+	u8	idreg_debugfs_iter;
+
 	/* Hypercall features firmware registers' descriptor */
 	struct kvm_smccc_features smccc_feat;
 	struct maple_tree smccc_filter;

arch/arm64/kvm/sys_regs.c

@@ -12,6 +12,7 @@
 #include <linux/bitfield.h>
 #include <linux/bsearch.h>
 #include <linux/cacheinfo.h>
+#include <linux/debugfs.h>
 #include <linux/kvm_host.h>
 #include <linux/mm.h>
 #include <linux/printk.h>
@@ -3422,6 +3423,81 @@ static bool emulate_sys_reg(struct kvm_vcpu *vcpu,
 	return false;
 }
 
+static void *idregs_debug_start(struct seq_file *s, loff_t *pos)
+{
+	struct kvm *kvm = s->private;
+	u8 *iter;
+
+	mutex_lock(&kvm->arch.config_lock);
+
+	iter = &kvm->arch.idreg_debugfs_iter;
+	if (*iter == (u8)~0) {
+		*iter = *pos;
+		if (*iter >= KVM_ARM_ID_REG_NUM)
+			iter = NULL;
+	} else {
+		iter = ERR_PTR(-EBUSY);
+	}
+
+	mutex_unlock(&kvm->arch.config_lock);
+
+	return iter;
+}
+
+static void *idregs_debug_next(struct seq_file *s, void *v, loff_t *pos)
+{
+	struct kvm *kvm = s->private;
+
+	(*pos)++;
+
+	if ((kvm->arch.idreg_debugfs_iter + 1) < KVM_ARM_ID_REG_NUM) {
+		kvm->arch.idreg_debugfs_iter++;
+
+		return &kvm->arch.idreg_debugfs_iter;
+	}
+
+	return NULL;
+}
+
+static void idregs_debug_stop(struct seq_file *s, void *v)
+{
+	struct kvm *kvm = s->private;
+
+	if (IS_ERR(v))
+		return;
+
+	mutex_lock(&kvm->arch.config_lock);
+
+	kvm->arch.idreg_debugfs_iter = ~0;
+
+	mutex_unlock(&kvm->arch.config_lock);
+}
+
+static int idregs_debug_show(struct seq_file *s, void *v)
+{
+	struct kvm *kvm = s->private;
+	const struct sys_reg_desc *desc;
+
+	desc = first_idreg + kvm->arch.idreg_debugfs_iter;
+
+	if (!desc->name)
+		return 0;
+
+	seq_printf(s, "%20s:\t%016llx\n",
+		   desc->name, IDREG(kvm, IDX_IDREG(kvm->arch.idreg_debugfs_iter)));
+
+	return 0;
+}
+
+static const struct seq_operations idregs_debug_sops = {
+	.start	= idregs_debug_start,
+	.next	= idregs_debug_next,
+	.stop	= idregs_debug_stop,
+	.show	= idregs_debug_show,
+};
+
+DEFINE_SEQ_ATTRIBUTE(idregs_debug);
+
 static void kvm_reset_id_regs(struct kvm_vcpu *vcpu)
 {
 	const struct sys_reg_desc *idreg = first_idreg;
@@ -3441,6 +3517,11 @@ static void kvm_reset_id_regs(struct kvm_vcpu *vcpu)
 		id = reg_to_encoding(idreg);
 	}
 
+	kvm->arch.idreg_debugfs_iter = ~0;
+
+	debugfs_create_file("idregs", 0444, kvm->debugfs_dentry, kvm,
+			    &idregs_debug_fops);
+
 	set_bit(KVM_ARCH_FLAG_ID_REGS_INITIALIZED, &kvm->arch.flags);
 }