[PATCH 2/2] KVM: riscv: Register ptdump with debugfs on guest creation

["Dylan.Wu" <fredwudi0305@gmail.com>]

Introduce KVM ptdump to show the guest gstage pagetables. This registers
a 'gstage_page_tables' file under the guest debugfs directory.
Userspace can now inspect the gstage layout and permissions, which
is useful for architectural debugging and memory management audits.

Assisted-by: YuanSheng: deepseek-v4-pro
Co-developed-by: Quan Zhou <zhouquan@iscas.ac.cn>
Signed-off-by: Quan Zhou <zhouquan@iscas.ac.cn>
Signed-off-by: Dylan.Wu <fredwudi0305@gmail.com>
---
 arch/riscv/include/asm/kvm_host.h |   6 +
 arch/riscv/kvm/Kconfig            |  15 +++
 arch/riscv/kvm/Makefile           |   1 +
 arch/riscv/kvm/ptdump.c           | 178 ++++++++++++++++++++++++++++++
 arch/riscv/kvm/vm.c               |   5 +
 5 files changed, 205 insertions(+)
 create mode 100644 arch/riscv/kvm/ptdump.c

diff --git a/arch/riscv/include/asm/kvm_host.h b/arch/riscv/include/asm/kvm_host.h
index 60017ceec..04129c5f8 100644
--- a/arch/riscv/include/asm/kvm_host.h
+++ b/arch/riscv/include/asm/kvm_host.h
@@ -322,4 +322,10 @@ void kvm_riscv_vcpu_record_steal_time(struct kvm_vcpu *vcpu);
 /* Flags representing implementation specific details */
 DECLARE_STATIC_KEY_FALSE(kvm_riscv_vsstage_tlb_no_gpa);
 
+#ifdef CONFIG_PTDUMP_GSTAGE_DEBUGFS
+void kvm_s2_ptdump_create_debugfs(struct kvm *kvm);
+#else
+static inline void kvm_s2_ptdump_create_debugfs(struct kvm *kvm) {}
+#endif
+
 #endif /* __RISCV_KVM_HOST_H__ */
diff --git a/arch/riscv/kvm/Kconfig b/arch/riscv/kvm/Kconfig
index ec2cee0a3..0ceb4a452 100644
--- a/arch/riscv/kvm/Kconfig
+++ b/arch/riscv/kvm/Kconfig
@@ -38,3 +38,18 @@ config KVM
 	  If unsure, say N.
 
 endif # VIRTUALIZATION
+
+config PTDUMP_GSTAGE_DEBUGFS
+	bool "Present the gstage pagetables to debugfs"
+	depends on KVM
+	depends on DEBUG_KERNEL
+	depends on DEBUG_FS
+	depends on PTDUMP_DEBUGFS
+	default n
+	help
+	  Say Y here if you want to show the RISC-V KVM gstage guest page tables
+	  layout in a debugfs file. This information is primarily useful for
+	  architecture-specific kernel developers and KVM maintainers to
+	  investigate memory mapping and permission issues. It is probably
+	  not a good idea to enable this feature in a production kernel.
+	  If in doubt, say N.
diff --git a/arch/riscv/kvm/Makefile b/arch/riscv/kvm/Makefile
index 296c2ba05..0170c8c3b 100644
--- a/arch/riscv/kvm/Makefile
+++ b/arch/riscv/kvm/Makefile
@@ -42,3 +42,4 @@ kvm-y += vcpu_timer.o
 kvm-y += vcpu_vector.o
 kvm-y += vm.o
 kvm-y += vmid.o
+kvm-$(CONFIG_PTDUMP_GSTAGE_DEBUGFS) += ptdump.o
diff --git a/arch/riscv/kvm/ptdump.c b/arch/riscv/kvm/ptdump.c
new file mode 100644
index 000000000..972d45d69
--- /dev/null
+++ b/arch/riscv/kvm/ptdump.c
@@ -0,0 +1,178 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Debug helper used to dump the gstage pagetables of the system.
+ */
+#include <linux/debugfs.h>
+#include <linux/kvm_host.h>
+#include <linux/seq_file.h>
+#include <linux/mm.h>
+#include <asm/ptdump.h>
+#include <asm/pgtable.h>
+
+static const struct ptdump_prot_bits gstage_pte_bits[] = {
+	{
+		.mask = _PAGE_SOFT,
+		.set = "RSW(%d)",
+		.clear = "  ..  ",
+	}, {
+		.mask = _PAGE_DIRTY,
+		.set = "D",
+		.clear = ".",
+	}, {
+		.mask = _PAGE_ACCESSED,
+		.set = "A",
+		.clear = ".",
+	}, {
+		.mask = _PAGE_USER,
+		.set = "U",
+		.clear = ".",
+	}, {
+		.mask = _PAGE_EXEC,
+		.set = "X",
+		.clear = ".",
+	}, {
+		.mask = _PAGE_WRITE,
+		.set = "W",
+		.clear = ".",
+	}, {
+		.mask = _PAGE_READ,
+		.set = "R",
+		.clear = ".",
+	}, {
+		.mask = _PAGE_PRESENT,
+		.set = "V",
+		.clear = ".",
+	}
+};
+
+static struct ptdump_pg_level gstage_pg_levels[] = {
+	{ .name = "PGD" },
+	{ .name = "P4D" },
+	{ .name = "PUD" },
+	{ .name = "PMD" },
+	{ .name = "PTE" },
+};
+
+struct kvm_ptdump_state {
+	struct kvm *kvm;
+	struct ptdump_pg_state parser_state;
+	struct addr_marker marker[2];
+	struct ptdump_range range[2];
+};
+
+static void kvm_ptdump_walk_level(struct ptdump_state *pt_st,
+				  unsigned long *tbl, int level,
+				  unsigned long start_addr)
+{
+	unsigned long addr = start_addr;
+	unsigned long next, virt_addr;
+	int i;
+	unsigned long step = 1UL << (PAGE_SHIFT + (4 - level) * 9);
+
+	for (i = 0; i < PTRS_PER_PTE; i++, addr += step) {
+		unsigned long val = tbl[i];
+
+		next = addr + step;
+
+		if (level == 4 || (val & _PAGE_LEAF) || !(val & _PAGE_PRESENT)) {
+			note_page(pt_st, addr, level, val);
+		} else {
+			unsigned long pa = (val >> _PAGE_PFN_SHIFT) << PAGE_SHIFT;
+
+			virt_addr = (unsigned long)phys_to_virt(pa);
+
+			kvm_ptdump_walk_level(pt_st, (unsigned long *)virt_addr,
+					      level + 1, addr);
+		}
+	}
+}
+
+static int kvm_ptdump_visitor(struct seq_file *m, void *v)
+{
+	struct kvm_ptdump_state *st = m->private;
+	struct kvm *kvm = st->kvm;
+	unsigned long *pgd = (unsigned long *)kvm->arch.pgd;
+	int start_level = 5 - kvm->arch.pgd_levels;
+	int i, j;
+
+	st->parser_state.level = -1;
+	st->parser_state.start_address = 0;
+	st->parser_state.seq = m;
+
+	for (i = 0; i < ARRAY_SIZE(gstage_pg_levels); i++) {
+		gstage_pg_levels[i].bits = gstage_pte_bits;
+		gstage_pg_levels[i].num = ARRAY_SIZE(gstage_pte_bits);
+		gstage_pg_levels[i].mask = 0;
+		for (j = 0; j < ARRAY_SIZE(gstage_pte_bits); j++)
+			gstage_pg_levels[i].mask |= gstage_pte_bits[j].mask;
+	}
+
+	read_lock(&kvm->mmu_lock);
+	if (pgd) {
+		kvm_ptdump_walk_level(&st->parser_state.ptdump, pgd,
+			start_level, 0);
+	}
+	read_unlock(&kvm->mmu_lock);
+
+	note_page(&st->parser_state.ptdump, 0, -1, 0);
+	return 0;
+}
+
+static int kvm_ptdump_open(struct inode *inode, struct file *file)
+{
+	struct kvm *kvm = inode->i_private;
+	struct kvm_ptdump_state *st;
+	int ret;
+
+	if (!kvm_get_kvm_safe(kvm))
+		return -ENOENT;
+
+	st = kzalloc(sizeof(*st), GFP_KERNEL);
+	if (!st) {
+		kvm_put_kvm(kvm);
+		return -ENOMEM;
+	}
+
+	st->kvm = kvm;
+	st->marker[0].name = "Guest IPA";
+	st->marker[0].start_address = 0;
+	st->marker[1].start_address = -1UL;
+	st->range[0].start = 0;
+	st->range[0].end = -1UL;
+
+	st->parser_state.marker = st->marker;
+	st->parser_state.pg_level = gstage_pg_levels;
+	st->parser_state.ptdump.range = st->range;
+
+	ret = single_open(file, kvm_ptdump_visitor, st);
+	if (ret) {
+		kfree(st);
+		kvm_put_kvm(kvm);
+	}
+	return ret;
+}
+
+static int kvm_ptdump_release(struct inode *inode, struct file *file)
+{
+	struct seq_file *seq = file->private_data;
+	struct kvm_ptdump_state *st = seq->private;
+	struct kvm *kvm = st->kvm;
+
+	kfree(st);
+	kvm_put_kvm(kvm);
+	return single_release(inode, file);
+}
+
+static const struct file_operations kvm_gstage_fops = {
+	.owner		= THIS_MODULE,
+	.open		= kvm_ptdump_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= kvm_ptdump_release,
+};
+
+void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
+{
+	debugfs_create_file("gstage_page_tables", 0400, kvm->debugfs_dentry, kvm,
+			    &kvm_gstage_fops);
+}
diff --git a/arch/riscv/kvm/vm.c b/arch/riscv/kvm/vm.c
index a9f083fee..464ad2eaf 100644
--- a/arch/riscv/kvm/vm.c
+++ b/arch/riscv/kvm/vm.c
@@ -269,3 +269,8 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 {
 	return -EINVAL;
 }
+
+void kvm_arch_create_vm_debugfs(struct kvm *kvm)
+{
+	kvm_s2_ptdump_create_debugfs(kvm);
+}
-- 
2.34.1