Subject: Re: [PATCH v2 2/2] KVM: arm64: nv: Expose shadow page tables in debugfs

From: Marc Zyngier <[email-masked]>
Date: Thu, 19 Mar 2026 16:01:51 +0000

On Tue, 17 Mar 2026 18:26:38 +0000,
Wei-Lin Chang <[email-masked]> 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 <[email-masked]>
> Reviewed-by: Sebastian Ene <[email-masked]>
> Signed-off-by: Wei-Lin Chang <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_host.h |  9 +++++++++
>  arch/arm64/include/asm/kvm_mmu.h  |  4 ++++
>  arch/arm64/kvm/nested.c           |  6 +++++-
>  arch/arm64/kvm/ptdump.c           | 27 +++++++++++++++++++++++++++
>  4 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 5d5a3bbdb95e..52977c9a11c3 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 +409,11 @@ struct kvm_arch {
>  	 * the associated pKVM instance in the hypervisor.
>  	 */
>  	struct kvm_protected_vm pkvm;
> +
> +#ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
> +	/* Nested virtualization info */
> +	struct dentry *debugfs_nv_dentry;
> +#endif
>  };
>  
>  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..aebbbad85d38 100644
> --- a/arch/arm64/kvm/ptdump.c
> +++ b/arch/arm64/kvm/ptdump.c
> @@ -10,12 +10,14 @@
>  #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>
>  
>  #define MARKERS_LEN		2
>  #define KVM_PGTABLE_MAX_LEVELS	(KVM_PGTABLE_LAST_LEVEL + 1)
> +#define S2FNAMESZ	sizeof("0x0123456789abcdef-0x0123456789abcdef-s2-disabled")
>  
>  struct kvm_ptdump_guest_state {
>  	struct kvm_s2_mmu	*mmu;
> @@ -277,6 +279,28 @@ 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;
> +	char file_name[S2FNAMESZ];
> +
> +	snprintf(file_name, sizeof(file_name), "0x%llx-0x%llx-s2-%sabled",
> +		 mmu->tlb_vttbr,
> +		 mmu->tlb_vtcr,
> +		 mmu->nested_stage2_enabled ? "en" : "dis");

Here's an example of this code in action:

root@semi-fraudulent:/home/maz/vminstall# ls -l /sys/kernel/debug/kvm/932-4/nested/
total 0
-r-------- 1 root root 0 Mar 19 11:58 0x0-0x80000000-s2-disabled
-r-------- 1 root root 0 Mar 19 12:01 0x100010124c000-0x800c3558-s2-enabled
-r-------- 1 root root 0 Mar 19 15:06 0x200010d916000-0x800c3558-s2-enabled
-r-------- 1 root root 0 Mar 19 15:16 0x30001027ec000-0x800c3558-s2-enabled

I think these hex values should always be padded with leading zeroes,
making them more readable.

I can hack that in as I queue the patches.

	M.

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