Exposing shadow page tables in debugfs improves the debugability and
testability of NV. With this patch a new directory "nested" is created
for each VM created if the host is NV capable. Within the directory each
valid s2 mmu will have its shadow page table exposed as a readable file
with the file name formatted as 0x<vttbr>-0x<vtcr>-s2-{en,dis}abled. The
creation and removal of the files happen at the points when an s2 mmu
becomes valid, or the context it represents change. In the future the
"nested" directory can also hold other NV related information.

This is gated behind CONFIG_PTDUMP_STAGE2_DEBUGFS.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Wei-Lin Chang <weilin.chang@arm.com>
---
 arch/arm64/include/asm/kvm_host.h |  7 +++++++
 arch/arm64/include/asm/kvm_mmu.h  |  4 ++++
 arch/arm64/kvm/nested.c           |  6 +++++-
 arch/arm64/kvm/ptdump.c           | 27 +++++++++++++++++++++++++++
 4 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 5d5a3bbdb95e..f88f6c4d646e 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -217,6 +217,10 @@ 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.
@@ -405,6 +409,9 @@ struct kvm_arch {
 	 * the associated pKVM instance in the hypervisor.
 	 */
 	struct kvm_protected_vm pkvm;
+
+	/* Nested virtualization info */
+	struct dentry *debugfs_nv_dentry;
 };
 
 struct kvm_vcpu_fault_info {
diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index d968aca0461a..01e9c72d6aa7 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -393,8 +393,12 @@ 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);
 #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) {}
 #endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
 
 #endif /* __ASSEMBLER__ */
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index eeea5e692370..31d74ed8449e 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -730,8 +730,10 @@ 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))
+	if (kvm_s2_mmu_valid(s2_mmu)) {
+		kvm_nested_s2_ptdump_remove_debugfs(s2_mmu);
 		s2_mmu->pending_unmap = true;
+	}
 
 	/*
 	 * The virtual VMID (modulo CnP) will be used as a key when matching
@@ -745,6 +747,8 @@ 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 98763b291956..3a70f633fc8b 100644
--- a/arch/arm64/kvm/ptdump.c
+++ b/arch/arm64/kvm/ptdump.c
@@ -10,6 +10,7 @@
 #include <linux/kvm_host.h>
 #include <linux/seq_file.h>
 
+#include <asm/cpufeature.h>
 #include <asm/kvm_mmu.h>
 #include <asm/kvm_pgtable.h>
 #include <asm/ptdump.h>
@@ -277,6 +278,29 @@ 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)
+{
+	struct dentry *dent;
+	/* format: 0x<vttbr>-0x<vtcr>-s2-{en, dis}abled\0 */
+	char file_name[2 + 16 + 1 + 2 + 16 + 4 + 3 + 6];
+
+	snprintf(file_name, sizeof(file_name), "0x%llx-0x%llx-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);
+
+	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);
+}
+
 void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
 {
 	debugfs_create_file("stage2_page_tables", 0400, kvm->debugfs_dentry,
@@ -285,4 +309,7 @@ void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
 			    &kvm->arch.mmu, &kvm_pgtable_range_fops);
 	debugfs_create_file("stage2_levels", 0400, kvm->debugfs_dentry,
 			    &kvm->arch.mmu, &kvm_pgtable_levels_fops);
+	if (cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
+		kvm->arch.debugfs_nv_dentry =
+			debugfs_create_dir("nested", kvm->debugfs_dentry);
 }
-- 
2.43.0

On Sun, Mar 08, 2026 at 11:18:29PM +0000, Wei-Lin Chang wrote:
> Exposing shadow page tables in debugfs improves the debugability and
> testability of NV. With this patch a new directory "nested" is created
> for each VM created if the host is NV capable. Within the directory each
> valid s2 mmu will have its shadow page table exposed as a readable file
> with the file name formatted as 0x<vttbr>-0x<vtcr>-s2-{en,dis}abled. The
> creation and removal of the files happen at the points when an s2 mmu
> becomes valid, or the context it represents change. In the future the
> "nested" directory can also hold other NV related information.
> 
> This is gated behind CONFIG_PTDUMP_STAGE2_DEBUGFS.
> 
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Wei-Lin Chang <weilin.chang@arm.com>
> ---
>  arch/arm64/include/asm/kvm_host.h |  7 +++++++
>  arch/arm64/include/asm/kvm_mmu.h  |  4 ++++
>  arch/arm64/kvm/nested.c           |  6 +++++-
>  arch/arm64/kvm/ptdump.c           | 27 +++++++++++++++++++++++++++
>  4 files changed, 43 insertions(+), 1 deletion(-)

Hi,

> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 5d5a3bbdb95e..f88f6c4d646e 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -217,6 +217,10 @@ 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.
> @@ -405,6 +409,9 @@ struct kvm_arch {
>  	 * the associated pKVM instance in the hypervisor.
>  	 */
>  	struct kvm_protected_vm pkvm;
> +
> +	/* Nested virtualization info */
> +	struct dentry *debugfs_nv_dentry;
>  };
>  
>  struct kvm_vcpu_fault_info {
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index d968aca0461a..01e9c72d6aa7 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -393,8 +393,12 @@ 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);
>  #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) {}
>  #endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
>  
>  #endif /* __ASSEMBLER__ */
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index eeea5e692370..31d74ed8449e 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -730,8 +730,10 @@ 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))
> +	if (kvm_s2_mmu_valid(s2_mmu)) {
> +		kvm_nested_s2_ptdump_remove_debugfs(s2_mmu);
>  		s2_mmu->pending_unmap = true;
> +	}
>  
>  	/*
>  	 * The virtual VMID (modulo CnP) will be used as a key when matching
> @@ -745,6 +747,8 @@ 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 98763b291956..3a70f633fc8b 100644
> --- a/arch/arm64/kvm/ptdump.c
> +++ b/arch/arm64/kvm/ptdump.c
> @@ -10,6 +10,7 @@
>  #include <linux/kvm_host.h>
>  #include <linux/seq_file.h>
>  
> +#include <asm/cpufeature.h>
>  #include <asm/kvm_mmu.h>
>  #include <asm/kvm_pgtable.h>
>  #include <asm/ptdump.h>
> @@ -277,6 +278,29 @@ 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)
> +{
> +	struct dentry *dent;
> +	/* format: 0x<vttbr>-0x<vtcr>-s2-{en, dis}abled\0 */
> +	char file_name[2 + 16 + 1 + 2 + 16 + 4 + 3 + 6];

Can you hide the size of this definition behind a macro ?

> +
> +	snprintf(file_name, sizeof(file_name), "0x%llx-0x%llx-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);
> +
> +	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);
> +}
> +
>  void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
>  {
>  	debugfs_create_file("stage2_page_tables", 0400, kvm->debugfs_dentry,
> @@ -285,4 +309,7 @@ void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
>  			    &kvm->arch.mmu, &kvm_pgtable_range_fops);
>  	debugfs_create_file("stage2_levels", 0400, kvm->debugfs_dentry,
>  			    &kvm->arch.mmu, &kvm_pgtable_levels_fops);
> +	if (cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
> +		kvm->arch.debugfs_nv_dentry =
> +			debugfs_create_dir("nested", kvm->debugfs_dentry);
>  }

Other than that this looks good to me, thanks
Reviewed-by: Sebastian Ene <sebastianene@google.com>

> -- 
> 2.43.0
>

On Wed, Mar 11, 2026 at 03:41:32PM +0000, Sebastian Ene wrote:
> On Sun, Mar 08, 2026 at 11:18:29PM +0000, Wei-Lin Chang wrote:
> > Exposing shadow page tables in debugfs improves the debugability and
> > testability of NV. With this patch a new directory "nested" is created
> > for each VM created if the host is NV capable. Within the directory each
> > valid s2 mmu will have its shadow page table exposed as a readable file
> > with the file name formatted as 0x<vttbr>-0x<vtcr>-s2-{en,dis}abled. The
> > creation and removal of the files happen at the points when an s2 mmu
> > becomes valid, or the context it represents change. In the future the
> > "nested" directory can also hold other NV related information.
> > 
> > This is gated behind CONFIG_PTDUMP_STAGE2_DEBUGFS.
> > 
> > Suggested-by: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Wei-Lin Chang <weilin.chang@arm.com>
> > ---
> >  arch/arm64/include/asm/kvm_host.h |  7 +++++++
> >  arch/arm64/include/asm/kvm_mmu.h  |  4 ++++
> >  arch/arm64/kvm/nested.c           |  6 +++++-
> >  arch/arm64/kvm/ptdump.c           | 27 +++++++++++++++++++++++++++
> >  4 files changed, 43 insertions(+), 1 deletion(-)
> 
> Hi,
> 
> > 
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 5d5a3bbdb95e..f88f6c4d646e 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -217,6 +217,10 @@ 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.
> > @@ -405,6 +409,9 @@ struct kvm_arch {
> >  	 * the associated pKVM instance in the hypervisor.
> >  	 */
> >  	struct kvm_protected_vm pkvm;
> > +
> > +	/* Nested virtualization info */
> > +	struct dentry *debugfs_nv_dentry;
> >  };
> >  
> >  struct kvm_vcpu_fault_info {
> > diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> > index d968aca0461a..01e9c72d6aa7 100644
> > --- a/arch/arm64/include/asm/kvm_mmu.h
> > +++ b/arch/arm64/include/asm/kvm_mmu.h
> > @@ -393,8 +393,12 @@ 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);
> >  #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) {}
> >  #endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
> >  
> >  #endif /* __ASSEMBLER__ */
> > diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> > index eeea5e692370..31d74ed8449e 100644
> > --- a/arch/arm64/kvm/nested.c
> > +++ b/arch/arm64/kvm/nested.c
> > @@ -730,8 +730,10 @@ 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))
> > +	if (kvm_s2_mmu_valid(s2_mmu)) {
> > +		kvm_nested_s2_ptdump_remove_debugfs(s2_mmu);
> >  		s2_mmu->pending_unmap = true;
> > +	}
> >  
> >  	/*
> >  	 * The virtual VMID (modulo CnP) will be used as a key when matching
> > @@ -745,6 +747,8 @@ 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 98763b291956..3a70f633fc8b 100644
> > --- a/arch/arm64/kvm/ptdump.c
> > +++ b/arch/arm64/kvm/ptdump.c
> > @@ -10,6 +10,7 @@
> >  #include <linux/kvm_host.h>
> >  #include <linux/seq_file.h>
> >  
> > +#include <asm/cpufeature.h>
> >  #include <asm/kvm_mmu.h>
> >  #include <asm/kvm_pgtable.h>
> >  #include <asm/ptdump.h>
> > @@ -277,6 +278,29 @@ 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)
> > +{
> > +	struct dentry *dent;
> > +	/* format: 0x<vttbr>-0x<vtcr>-s2-{en, dis}abled\0 */
> > +	char file_name[2 + 16 + 1 + 2 + 16 + 4 + 3 + 6];
> 
> Can you hide the size of this definition behind a macro ?

Sure, no problem.

> 
> > +
> > +	snprintf(file_name, sizeof(file_name), "0x%llx-0x%llx-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);
> > +
> > +	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);
> > +}
> > +
> >  void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
> >  {
> >  	debugfs_create_file("stage2_page_tables", 0400, kvm->debugfs_dentry,
> > @@ -285,4 +309,7 @@ void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
> >  			    &kvm->arch.mmu, &kvm_pgtable_range_fops);
> >  	debugfs_create_file("stage2_levels", 0400, kvm->debugfs_dentry,
> >  			    &kvm->arch.mmu, &kvm_pgtable_levels_fops);
> > +	if (cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
> > +		kvm->arch.debugfs_nv_dentry =
> > +			debugfs_create_dir("nested", kvm->debugfs_dentry);
> >  }
> 
> Other than that this looks good to me, thanks
> Reviewed-by: Sebastian Ene <sebastianene@google.com>

Thanks for the review!

Thanks,
Wei-Lin Chang
> 
> > -- 
> > 2.43.0
> >

On Fri, 13 Mar 2026 22:28:38 +0000,
Wei-Lin Chang <weilin.chang@arm.com> wrote:
> 
> On Wed, Mar 11, 2026 at 03:41:32PM +0000, Sebastian Ene wrote:
> > On Sun, Mar 08, 2026 at 11:18:29PM +0000, Wei-Lin Chang wrote:

[...]

> > > +void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu)
> > > +{
> > > +	struct dentry *dent;
> > > +	/* format: 0x<vttbr>-0x<vtcr>-s2-{en, dis}abled\0 */
> > > +	char file_name[2 + 16 + 1 + 2 + 16 + 4 + 3 + 6];
> > 
> > Can you hide the size of this definition behind a macro ?
> 
> Sure, no problem.

I don't think a macro helps that much on its own. I'd rather this is
made a bit more foolproof by directly expressing the actual string
this results into, and use that for the size:

  #define S2FNAMESZ sizeof("0x0123456789abcdef-0x0123456789abcdef-s2-disabled")

  char file_name[S2FNAME];

which is less ambiguous. The compiler will happily optimise this to a
nice constant that also accounts for the final '\0'.

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.

On Sat, Mar 14, 2026 at 09:39:17AM +0000, Marc Zyngier wrote:
> On Fri, 13 Mar 2026 22:28:38 +0000,
> Wei-Lin Chang <weilin.chang@arm.com> wrote:
> > 
> > On Wed, Mar 11, 2026 at 03:41:32PM +0000, Sebastian Ene wrote:
> > > On Sun, Mar 08, 2026 at 11:18:29PM +0000, Wei-Lin Chang wrote:
> 
> [...]
> 
> > > > +void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu)
> > > > +{
> > > > +	struct dentry *dent;
> > > > +	/* format: 0x<vttbr>-0x<vtcr>-s2-{en, dis}abled\0 */
> > > > +	char file_name[2 + 16 + 1 + 2 + 16 + 4 + 3 + 6];
> > > 
> > > Can you hide the size of this definition behind a macro ?
> > 
> > Sure, no problem.
> 
> I don't think a macro helps that much on its own. I'd rather this is
> made a bit more foolproof by directly expressing the actual string
> this results into, and use that for the size:
> 
>   #define S2FNAMESZ sizeof("0x0123456789abcdef-0x0123456789abcdef-s2-disabled")
> 
>   char file_name[S2FNAME];
> 
> which is less ambiguous. The compiler will happily optimise this to a
> nice constant that also accounts for the final '\0'.

Thanks for the advice!
Makes sense to me, I'll change it to that.

Thanks,
Wei-Lin Chang

> 
> Thanks,
> 
> 	M.
> 
> -- 
> Jazz isn't dead. It just smells funny.

On Sun, Mar 08, 2026 at 11:18:29PM +0000, Wei-Lin Chang wrote:
> Exposing shadow page tables in debugfs improves the debugability and
> testability of NV. With this patch a new directory "nested" is created
> for each VM created if the host is NV capable. Within the directory each
> valid s2 mmu will have its shadow page table exposed as a readable file
> with the file name formatted as 0x<vttbr>-0x<vtcr>-s2-{en,dis}abled. The
> creation and removal of the files happen at the points when an s2 mmu
> becomes valid, or the context it represents change. In the future the
> "nested" directory can also hold other NV related information.
> 
> This is gated behind CONFIG_PTDUMP_STAGE2_DEBUGFS.
> 
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Wei-Lin Chang <weilin.chang@arm.com>
> ---
>  arch/arm64/include/asm/kvm_host.h |  7 +++++++
>  arch/arm64/include/asm/kvm_mmu.h  |  4 ++++
>  arch/arm64/kvm/nested.c           |  6 +++++-
>  arch/arm64/kvm/ptdump.c           | 27 +++++++++++++++++++++++++++
>  4 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 5d5a3bbdb95e..f88f6c4d646e 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -217,6 +217,10 @@ 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.
> @@ -405,6 +409,9 @@ struct kvm_arch {
>  	 * the associated pKVM instance in the hypervisor.
>  	 */
>  	struct kvm_protected_vm pkvm;
> +
> +	/* Nested virtualization info */
> +	struct dentry *debugfs_nv_dentry;

Seems like this could be guarded with CONFIG_PTDUMP_STAGE2_DEBUGFS too, or is
there a reason you didn't?

Thanks,
Joey

>  };
>  
>  struct kvm_vcpu_fault_info {
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index d968aca0461a..01e9c72d6aa7 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -393,8 +393,12 @@ 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);
>  #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) {}
>  #endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
>  
>  #endif /* __ASSEMBLER__ */
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index eeea5e692370..31d74ed8449e 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -730,8 +730,10 @@ 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))
> +	if (kvm_s2_mmu_valid(s2_mmu)) {
> +		kvm_nested_s2_ptdump_remove_debugfs(s2_mmu);
>  		s2_mmu->pending_unmap = true;
> +	}
>  
>  	/*
>  	 * The virtual VMID (modulo CnP) will be used as a key when matching
> @@ -745,6 +747,8 @@ 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 98763b291956..3a70f633fc8b 100644
> --- a/arch/arm64/kvm/ptdump.c
> +++ b/arch/arm64/kvm/ptdump.c
> @@ -10,6 +10,7 @@
>  #include <linux/kvm_host.h>
>  #include <linux/seq_file.h>
>  
> +#include <asm/cpufeature.h>
>  #include <asm/kvm_mmu.h>
>  #include <asm/kvm_pgtable.h>
>  #include <asm/ptdump.h>
> @@ -277,6 +278,29 @@ 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)
> +{
> +	struct dentry *dent;
> +	/* format: 0x<vttbr>-0x<vtcr>-s2-{en, dis}abled\0 */
> +	char file_name[2 + 16 + 1 + 2 + 16 + 4 + 3 + 6];
> +
> +	snprintf(file_name, sizeof(file_name), "0x%llx-0x%llx-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);
> +
> +	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);
> +}
> +
>  void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
>  {
>  	debugfs_create_file("stage2_page_tables", 0400, kvm->debugfs_dentry,
> @@ -285,4 +309,7 @@ void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
>  			    &kvm->arch.mmu, &kvm_pgtable_range_fops);
>  	debugfs_create_file("stage2_levels", 0400, kvm->debugfs_dentry,
>  			    &kvm->arch.mmu, &kvm_pgtable_levels_fops);
> +	if (cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
> +		kvm->arch.debugfs_nv_dentry =
> +			debugfs_create_dir("nested", kvm->debugfs_dentry);
>  }
> -- 
> 2.43.0
>

On Tue, Mar 10, 2026 at 03:26:04PM +0000, Joey Gouly wrote:
> On Sun, Mar 08, 2026 at 11:18:29PM +0000, Wei-Lin Chang wrote:
> > Exposing shadow page tables in debugfs improves the debugability and
> > testability of NV. With this patch a new directory "nested" is created
> > for each VM created if the host is NV capable. Within the directory each
> > valid s2 mmu will have its shadow page table exposed as a readable file
> > with the file name formatted as 0x<vttbr>-0x<vtcr>-s2-{en,dis}abled. The
> > creation and removal of the files happen at the points when an s2 mmu
> > becomes valid, or the context it represents change. In the future the
> > "nested" directory can also hold other NV related information.
> > 
> > This is gated behind CONFIG_PTDUMP_STAGE2_DEBUGFS.
> > 
> > Suggested-by: Marc Zyngier <maz@kernel.org>
> > Signed-off-by: Wei-Lin Chang <weilin.chang@arm.com>
> > ---
> >  arch/arm64/include/asm/kvm_host.h |  7 +++++++
> >  arch/arm64/include/asm/kvm_mmu.h  |  4 ++++
> >  arch/arm64/kvm/nested.c           |  6 +++++-
> >  arch/arm64/kvm/ptdump.c           | 27 +++++++++++++++++++++++++++
> >  4 files changed, 43 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 5d5a3bbdb95e..f88f6c4d646e 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -217,6 +217,10 @@ 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.
> > @@ -405,6 +409,9 @@ struct kvm_arch {
> >  	 * the associated pKVM instance in the hypervisor.
> >  	 */
> >  	struct kvm_protected_vm pkvm;
> > +
> > +	/* Nested virtualization info */
> > +	struct dentry *debugfs_nv_dentry;
> 
> Seems like this could be guarded with CONFIG_PTDUMP_STAGE2_DEBUGFS too, or is
> there a reason you didn't?

Right, I was thinking this "nested" directory can also hold other NV
related information in the future, that's why I didn't keep it behind
the option.

Thanks,
Wei-Lin Chang

> 
> Thanks,
> Joey
> 
> >  };
> >  
> >  struct kvm_vcpu_fault_info {
> > diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> > index d968aca0461a..01e9c72d6aa7 100644
> > --- a/arch/arm64/include/asm/kvm_mmu.h
> > +++ b/arch/arm64/include/asm/kvm_mmu.h
> > @@ -393,8 +393,12 @@ 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);
> >  #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) {}
> >  #endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
> >  
> >  #endif /* __ASSEMBLER__ */
> > diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> > index eeea5e692370..31d74ed8449e 100644
> > --- a/arch/arm64/kvm/nested.c
> > +++ b/arch/arm64/kvm/nested.c
> > @@ -730,8 +730,10 @@ 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))
> > +	if (kvm_s2_mmu_valid(s2_mmu)) {
> > +		kvm_nested_s2_ptdump_remove_debugfs(s2_mmu);
> >  		s2_mmu->pending_unmap = true;
> > +	}
> >  
> >  	/*
> >  	 * The virtual VMID (modulo CnP) will be used as a key when matching
> > @@ -745,6 +747,8 @@ 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 98763b291956..3a70f633fc8b 100644
> > --- a/arch/arm64/kvm/ptdump.c
> > +++ b/arch/arm64/kvm/ptdump.c
> > @@ -10,6 +10,7 @@
> >  #include <linux/kvm_host.h>
> >  #include <linux/seq_file.h>
> >  
> > +#include <asm/cpufeature.h>
> >  #include <asm/kvm_mmu.h>
> >  #include <asm/kvm_pgtable.h>
> >  #include <asm/ptdump.h>
> > @@ -277,6 +278,29 @@ 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)
> > +{
> > +	struct dentry *dent;
> > +	/* format: 0x<vttbr>-0x<vtcr>-s2-{en, dis}abled\0 */
> > +	char file_name[2 + 16 + 1 + 2 + 16 + 4 + 3 + 6];
> > +
> > +	snprintf(file_name, sizeof(file_name), "0x%llx-0x%llx-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);
> > +
> > +	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);
> > +}
> > +
> >  void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
> >  {
> >  	debugfs_create_file("stage2_page_tables", 0400, kvm->debugfs_dentry,
> > @@ -285,4 +309,7 @@ void kvm_s2_ptdump_create_debugfs(struct kvm *kvm)
> >  			    &kvm->arch.mmu, &kvm_pgtable_range_fops);
> >  	debugfs_create_file("stage2_levels", 0400, kvm->debugfs_dentry,
> >  			    &kvm->arch.mmu, &kvm_pgtable_levels_fops);
> > +	if (cpus_have_final_cap(ARM64_HAS_NESTED_VIRT))
> > +		kvm->arch.debugfs_nv_dentry =
> > +			debugfs_create_dir("nested", kvm->debugfs_dentry);
> >  }
> > -- 
> > 2.43.0
> >

On Tue, 10 Mar 2026 16:17:07 +0000,
Wei-Lin Chang <weilin.chang@arm.com> wrote:
> 
> On Tue, Mar 10, 2026 at 03:26:04PM +0000, Joey Gouly wrote:

> > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > index 5d5a3bbdb95e..f88f6c4d646e 100644
> > > --- a/arch/arm64/include/asm/kvm_host.h
> > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > @@ -217,6 +217,10 @@ 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.
> > > @@ -405,6 +409,9 @@ struct kvm_arch {
> > >  	 * the associated pKVM instance in the hypervisor.
> > >  	 */
> > >  	struct kvm_protected_vm pkvm;
> > > +
> > > +	/* Nested virtualization info */
> > > +	struct dentry *debugfs_nv_dentry;
> > 
> > Seems like this could be guarded with CONFIG_PTDUMP_STAGE2_DEBUGFS too, or is
> > there a reason you didn't?
> 
> Right, I was thinking this "nested" directory can also hold other NV
> related information in the future, that's why I didn't keep it behind
> the option.

We can cross that bridge when we get there. In the meantime, I'd side
with Joey and would rather not have a dangling pointer.

Thanks,

	M.

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

On Tue, Mar 10, 2026 at 04:50:22PM +0000, Marc Zyngier wrote:
> On Tue, 10 Mar 2026 16:17:07 +0000,
> Wei-Lin Chang <weilin.chang@arm.com> wrote:
> > 
> > On Tue, Mar 10, 2026 at 03:26:04PM +0000, Joey Gouly wrote:
> 
> > > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > > > index 5d5a3bbdb95e..f88f6c4d646e 100644
> > > > --- a/arch/arm64/include/asm/kvm_host.h
> > > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > > @@ -217,6 +217,10 @@ 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.
> > > > @@ -405,6 +409,9 @@ struct kvm_arch {
> > > >  	 * the associated pKVM instance in the hypervisor.
> > > >  	 */
> > > >  	struct kvm_protected_vm pkvm;
> > > > +
> > > > +	/* Nested virtualization info */
> > > > +	struct dentry *debugfs_nv_dentry;
> > > 
> > > Seems like this could be guarded with CONFIG_PTDUMP_STAGE2_DEBUGFS too, or is
> > > there a reason you didn't?
> > 
> > Right, I was thinking this "nested" directory can also hold other NV
> > related information in the future, that's why I didn't keep it behind
> > the option.
> 
> We can cross that bridge when we get there. In the meantime, I'd side
> with Joey and would rather not have a dangling pointer.

Sure!
Let me guard it with CONFIG_PTDUMP_STAGE2_DEBUGFS too.

Thanks,
Wei-Lin Chang

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