[PATCH v3 2/2] KVM: arm64: ptdump: Fix shadow ptdump sleep-in-atomic-context problem
From: Wei-Lin Chang
Date: Wed Sep 16 2026 - 19:05:21 EST
Previously we exposed shadow page tables by creating a debugfs ptdump
file whenever a nested mmu instance gets bound to a new context, and
deleting the debugfs file whose context was getting unbound.
This turned out to be buggy, as the instance<->context binding process
is done with the mmu_lock held, and debugfs creation/deletion can sleep.
Change the approach and create a shadow ptdump file for each nested mmu
instead during kvm_vcpu_init_nested(), in a sleepable context. The files
will be named nested_mmu<index>, and reading it will return the nested
context's VTTBR, VTCR, and s2 enabled or not before dumping the shadow
page tables, given the nested mmu is valid.
Fixes: 19e15dc73f0f ("KVM: arm64: nv: Expose shadow page tables in debugfs")
Reported-by: Itaru Kitayama <itaru.kitayama@xxxxxxxxxxx>
Closes: https://lore.kernel.org/kvmarm/aiuF0KSvvv-ZozI1@sm-arm-grace07/
Signed-off-by: Wei-Lin Chang <weilin.chang@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
arch/arm64/include/asm/kvm_host.h | 4 ----
arch/arm64/include/asm/kvm_mmu.h | 6 ++----
arch/arm64/kvm/nested.c | 9 ++++-----
arch/arm64/kvm/ptdump.c | 32 +++++++++++++++----------------
4 files changed, 21 insertions(+), 30 deletions(-)
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index cd9b9d2462f9..db177af9149f 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -217,10 +217,6 @@ struct kvm_s2_mmu {
*/
bool nested_stage2_enabled;
-#ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
- struct dentry *shadow_pt_debugfs_dentry;
-#endif
-
/*
* true when this MMU needs to be unmapped before being used for a new
* purpose.
diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 6eae7e7e2a68..ac2b0637692a 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -392,12 +392,10 @@ static inline bool kvm_supports_cacheable_pfnmap(void)
#ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
void kvm_s2_ptdump_create_debugfs(struct kvm *kvm);
-void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu);
-void kvm_nested_s2_ptdump_remove_debugfs(struct kvm_s2_mmu *mmu);
+void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu, int idx);
#else
static inline void kvm_s2_ptdump_create_debugfs(struct kvm *kvm) {}
-static inline void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu) {}
-static inline void kvm_nested_s2_ptdump_remove_debugfs(struct kvm_s2_mmu *mmu) {}
+static inline void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu, int idx) {}
#endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
#endif /* __ASSEMBLER__ */
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index b191365d97cc..41253fe45941 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -124,6 +124,9 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
return ret;
}
+ for (i = 0; i < S2_MMU_PER_VCPU; i++)
+ kvm_nested_s2_ptdump_create_debugfs(&tmp[i], i + kvm->arch.nested_mmus_size);
+
guard(write_lock)(&kvm->mmu_lock);
for (i = 0; i < S2_MMU_PER_VCPU; i++)
@@ -837,10 +840,8 @@ static struct kvm_s2_mmu *get_s2_mmu_nested(struct kvm_vcpu *vcpu)
kvm->arch.nested_mmus_next = (i + 1) % kvm->arch.nested_mmus_size;
/* Make sure we don't forget to do the laundry */
- if (kvm_s2_mmu_valid(s2_mmu)) {
- kvm_nested_s2_ptdump_remove_debugfs(s2_mmu);
+ if (kvm_s2_mmu_valid(s2_mmu))
s2_mmu->pending_unmap = true;
- }
/*
* The virtual VMID (modulo CnP) will be used as a key when matching
@@ -854,8 +855,6 @@ static struct kvm_s2_mmu *get_s2_mmu_nested(struct kvm_vcpu *vcpu)
s2_mmu->tlb_vtcr = vcpu_read_sys_reg(vcpu, VTCR_EL2);
s2_mmu->nested_stage2_enabled = vcpu_read_sys_reg(vcpu, HCR_EL2) & HCR_VM;
- kvm_nested_s2_ptdump_create_debugfs(s2_mmu);
-
out:
atomic_inc(&s2_mmu->refcnt);
diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
index 4ead856c181a..9411d6c2dafb 100644
--- a/arch/arm64/kvm/ptdump.c
+++ b/arch/arm64/kvm/ptdump.c
@@ -17,7 +17,7 @@
#define MARKERS_LEN 2
#define KVM_PGTABLE_MAX_LEVELS (KVM_PGTABLE_LAST_LEVEL + 1)
-#define S2FNAMESZ sizeof("0x0123456789abcdef-0x0123456789abcdef-s2-disabled")
+#define S2FNAMESZ sizeof("nested_mmu0000")
struct kvm_ptdump_guest_state {
struct kvm_s2_mmu *mmu;
@@ -173,6 +173,15 @@ static int kvm_ptdump_guest_show(struct seq_file *m, void *unused)
.seq = m,
};
+ if (kvm_is_nested_s2_mmu(kvm, mmu)) {
+ if (kvm_s2_mmu_valid(mmu))
+ seq_printf(m, "VTCR: 0x%016llx VTTBR: 0x%016llx s2: %s\n",
+ mmu->tlb_vtcr, mmu->tlb_vttbr,
+ mmu->nested_stage2_enabled ? "enabled" : "disabled");
+ else
+ return 0;
+ }
+
ret = kvm_pgtable_walk(mmu->pgt, 0, BIT(mmu->pgt->ia_bits), &walker);
if (ret)
return ret;
@@ -299,26 +308,15 @@ static const struct file_operations kvm_pgtable_levels_fops = {
.release = kvm_pgtable_debugfs_close,
};
-void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu)
+void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu, int idx)
{
- struct dentry *dent;
char file_name[S2FNAMESZ];
- snprintf(file_name, sizeof(file_name), "0x%016llx-0x%016llx-s2-%sabled",
- mmu->tlb_vttbr,
- mmu->tlb_vtcr,
- mmu->nested_stage2_enabled ? "en" : "dis");
-
- dent = debugfs_create_file(file_name, 0400,
- mmu->arch->debugfs_nv_dentry, mmu,
- &kvm_ptdump_guest_fops);
+ snprintf(file_name, sizeof(file_name), "nested_mmu%d", idx);
- mmu->shadow_pt_debugfs_dentry = dent;
-}
-
-void kvm_nested_s2_ptdump_remove_debugfs(struct kvm_s2_mmu *mmu)
-{
- debugfs_remove(mmu->shadow_pt_debugfs_dentry);
+ debugfs_create_file(file_name, 0400,
+ mmu->arch->debugfs_nv_dentry, mmu,
+ &kvm_ptdump_guest_fops);
}
void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
--
2.43.0