On Tue, Aug 20, 2024 at 03:06:45PM +0100, Marc Zyngier wrote:

Hello Marc,

On Fri, 16 Aug 2024 13:39:04 +0100,
Sebastian Ene [off-list ref] wrote:

quoted

While arch/*/mem/ptdump handles the kernel pagetable dumping code,
introduce KVM/ptdump which shows the guest stage-2 pagetables. The
separation is necessary because most of the definitions from the
stage-2 pagetable reside in the KVM path and we will be invoking
functionality **specific** to KVM.

Drop the ** emphasis.

I will drop this.

quoted

When a guest is created, register a new file entry under the guest
debugfs dir which allows userspace to show the contents of the guest
stage-2 pagetables when accessed.

Signed-off-by: Sebastian Ene <redacted>
---
 arch/arm64/kvm/Kconfig      | 14 ++++++
 arch/arm64/kvm/Makefile     |  1 +
 arch/arm64/kvm/arm.c        |  2 +
 arch/arm64/kvm/kvm_ptdump.h | 20 ++++++++
 arch/arm64/kvm/ptdump.c     | 91 +++++++++++++++++++++++++++++++++++++
 5 files changed, 128 insertions(+)
 create mode 100644 arch/arm64/kvm/kvm_ptdump.h
 create mode 100644 arch/arm64/kvm/ptdump.c

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 8304eb342be9..fcc41e58ede6 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig

@@ -66,4 +66,18 @@ config PROTECTED_NVHE_STACKTRACE
 
 	  If unsure, or not using protected nVHE (pKVM), say N.
 
+config PTDUMP_STAGE2_DEBUGFS
+       bool "Present the stage-2 pagetables to debugfs"
+       depends on KVM
+       select PTDUMP_CORE

This looks wrong. Looking at PTDUMP_DEBUGFS, it has the following
constraints:

        depends on DEBUG_KERNEL
        depends on DEBUG_FS
        depends on GENERIC_PTDUMP
        select PTDUMP_CORE

I don't see why the Stage-2 version should have anything different.

quoted

+       default n
+       help
+         Say Y here if you want to show the stage-2 kernel pagetables
+         layout in a debugfs file. This information is only useful for kernel developers
+         who are working in architecture specific areas of the kernel.
+         It is probably not a good idea to enable this feature in a production
+         kernel.

nit: try to keep the formatting within 80 columns.

Let me fix the formatting and add the other depends on rules.

More importantly, I find it very strange to expose the configuration
option so early in the series, while the support code isn't there yet.
You can perfectly introduce code that is conditional on a config
option and only add it at the end.

I will move the configuration option as a separate patch in the last
part of the series.

quoted

+
+         If in doubt, say N.
+
 endif # VIRTUALIZATION

diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
index 86a629aaf0a1..e4233b323a73 100644
--- a/arch/arm64/kvm/Makefile
+++ b/arch/arm64/kvm/Makefile

@@ -27,6 +27,7 @@ kvm-y += arm.o mmu.o mmio.o psci.o hypercalls.o pvtime.o \
 
 kvm-$(CONFIG_HW_PERF_EVENTS)  += pmu-emul.o pmu.o
 kvm-$(CONFIG_ARM64_PTR_AUTH)  += pauth.o
+kvm-$(CONFIG_PTDUMP_STAGE2_DEBUGFS) += ptdump.o
 
 always-y := hyp_constants.h hyp-constants.s
 

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 9bef7638342e..60fed2146763 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c

@@ -45,6 +45,7 @@
 #include <kvm/arm_hypercalls.h>
 #include <kvm/arm_pmu.h>
 #include <kvm/arm_psci.h>
+#include <kvm_ptdump.h>
 
 static enum kvm_mode kvm_mode = KVM_MODE_DEFAULT;
 

@@ -228,6 +229,7 @@ vm_fault_t kvm_arch_vcpu_fault(struct kvm_vcpu *vcpu, struct vm_fault *vmf)
 void kvm_arch_create_vm_debugfs(struct kvm *kvm)
 {
 	kvm_sys_regs_create_debugfs(kvm);
+	kvm_ptdump_guest_register(kvm);

Consider using a name that is homogeneous with what we already have
(kvm_s2_ptdump_create_debugfs?).

quoted

 }
 
 static void kvm_destroy_mpidr_data(struct kvm *kvm)

diff --git a/arch/arm64/kvm/kvm_ptdump.h b/arch/arm64/kvm/kvm_ptdump.h
new file mode 100644
index 000000000000..0a62b0e2908c
--- /dev/null
+++ b/arch/arm64/kvm/kvm_ptdump.h

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) Google, 2024
+ * Author: Sebastian Ene <sebastianene@google.com>
+ */
+
+#ifndef __KVM_PTDUMP_H
+#define __KVM_PTDUMP_H
+
+#include <linux/kvm_host.h>
+#include <asm/ptdump.h>
+
+
+#ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
+void kvm_ptdump_guest_register(struct kvm *kvm);
+#else
+static inline void kvm_ptdump_guest_register(struct kvm *kvm) {}
+#endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
+
+#endif /* __KVM_PTDUMP_H */

Please don't add new include files that contain so little stuff. These
things may as well be added either to asm/kvm_host.h or asm/ptdump.h.

quoted

diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
new file mode 100644
index 000000000000..52483d56be2e
--- /dev/null
+++ b/arch/arm64/kvm/ptdump.c

@@ -0,0 +1,91 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Debug helper used to dump the stage-2 pagetables of the system and their
+ * associated permissions.
+ *
+ * Copyright (C) Google, 2024
+ * Author: Sebastian Ene <sebastianene@google.com>
+ */
+#include <linux/debugfs.h>
+#include <linux/kvm_host.h>
+#include <linux/seq_file.h>
+
+#include <asm/kvm_pgtable.h>
+#include <kvm_ptdump.h>
+
+
+static int kvm_ptdump_visitor(const struct kvm_pgtable_visit_ctx *ctx,
+			      enum kvm_pgtable_walk_flags visit)
+{
+	struct ptdump_pg_state *st = ctx->arg;
+	struct ptdump_state *pt_st = &st->ptdump;
+
+	note_page(pt_st, ctx->addr, ctx->level, ctx->old);
+	return 0;
+}
+
+static int kvm_ptdump_show_common(struct seq_file *m,

What does "common" mean here? You have exactly *one* caller, so why
isn't that inlined in kvm_ptdump_guest_show()?

quoted

+				  struct kvm_pgtable *pgtable,
+				  struct ptdump_pg_state *parser_state)
+{
+	struct kvm_pgtable_walker walker = (struct kvm_pgtable_walker) {
+		.cb     = kvm_ptdump_visitor,
+		.arg	= parser_state,
+		.flags	= KVM_PGTABLE_WALK_LEAF,
+	};
+
+	parser_state->level = -1;
+	parser_state->start_address = 0;
+
+	return kvm_pgtable_walk(pgtable, 0, BIT(pgtable->ia_bits), &walker);
+}
+
+static int kvm_ptdump_guest_show(struct seq_file *m, void *unused)
+{
+	struct kvm *kvm = m->private;
+	struct kvm_s2_mmu *mmu = &kvm->arch.mmu;
+	struct ptdump_pg_state parser_state = {0};

nit: the common idiom is "parser_state = {}".

quoted

+	int ret;
+
+	write_lock(&kvm->mmu_lock);
+	ret = kvm_ptdump_show_common(m, mmu->pgt, &parser_state);
+	write_unlock(&kvm->mmu_lock);
+
+	return ret;
+}
+
+static int kvm_ptdump_guest_open(struct inode *m, struct file *file)
+{
+	struct kvm *kvm = m->i_private;
+	int ret;
+
+	if (!kvm_get_kvm_safe(kvm))
+		return -ENOENT;
+
+	ret = single_open(file, kvm_ptdump_guest_show, m->i_private);
+	if (ret < 0)
+		kvm_put_kvm(kvm);
+
+	return ret;
+}
+
+static int kvm_ptdump_guest_close(struct inode *m, struct file *file)
+{
+	struct kvm *kvm = m->i_private;
+
+	kvm_put_kvm(kvm);
+	return single_release(m, file);
+}
+
+static const struct file_operations kvm_ptdump_guest_fops = {
+	.open		= kvm_ptdump_guest_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= kvm_ptdump_guest_close,
+};
+
+void kvm_ptdump_guest_register(struct kvm *kvm)
+{
+	debugfs_create_file("stage2_page_tables", 0400, kvm->debugfs_dentry,
+			    kvm, &kvm_ptdump_guest_fops);
+}
-- 
2.46.0.184.g6999bdac58-goog

Overall, I have a hard time understanding what this does. It walks the
page tables, but doesn't do anything useful.

I have the feeling that this patch would be better squashed together
with patch #5, since it significantly reworks what patch #4 does.

Right, I think there are a few patches that should be squashed to
prevent this "empty" functionality.

Thanks for the feedback,
Seb

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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