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

[Itaru Kitayama <itaru.kitayama@fujitsu.com>]

Hi Wei Lin,
On Tue, Mar 17, 2026 at 06:26:38PM +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>
> Reviewed-by: Sebastian Ene <sebastianene@google.com>
> Signed-off-by: Wei-Lin Chang <weilin.chang@arm.com>
> ---
>  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);
> +

This function can sleep, so I get while running your shadow stage 2 KVM
selftest a messge:

[ 4408.411009] BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:1624
[ 4408.411075] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 164, name: shadow_stage2
[ 4408.411136] preempt_count: 2, expected: 0
[ 4408.411172] RCU nest depth: 0, expected: 0
[ 4408.411228] CPU: 1 UID: 0 PID: 164 Comm: shadow_stage2 Tainted: G        W           7.1.0-rc2+ #48 PREEMPT(full)
[ 4408.411336] Tainted: [W]=WARN
[ 4408.411368] Hardware name:  , BIOS
[ 4408.411403] Call trace:
[ 4408.411427]  show_stack+0x24/0x50 (C)
[ 4408.411524]  dump_stack_lvl+0x90/0x158
[ 4408.411633]  dump_stack+0x1c/0x38
[ 4408.411741]  __might_resched+0x168/0x208
[ 4408.411839]  __might_sleep+0x54/0xb0
[ 4408.411936]  down_write+0x30/0xe8
[ 4408.412048]  start_dirop+0x3c/0xc0
[ 4408.412149]  simple_start_creating+0xb8/0xc8
[ 4408.412241]  debugfs_start_creating.part.0+0x68/0x180
[ 4408.412375]  __debugfs_create_file+0x80/0x1f8
[ 4408.412505]  debugfs_create_file_full+0x28/0x68
[ 4408.412637]  kvm_nested_s2_ptdump_create_debugfs+0xa0/0x108
[ 4408.412734]  kvm_vcpu_load_hw_mmu+0x27c/0x320
[ 4408.412839]  kvm_arch_vcpu_load+0x318/0x5a0
[ 4408.412971]  kvm_emulate_nested_eret+0x148/0x3d8
[ 4408.413072]  kvm_handle_eret+0x110/0x138
[ 4408.413190]  handle_exit+0x6c/0x1e8
[ 4408.413306]  kvm_arch_vcpu_ioctl_run+0x3c4/0xc90
[ 4408.413396]  kvm_vcpu_ioctl+0x1a0/0xa68
[ 4408.413508]  __arm64_sys_ioctl+0xd0/0x160
[L1] L2 exit[ 4408.413631]  invoke_syscall+0xa8/0x138
[ 4408.413723]  el0_svc_common.constprop.0+0x4c/0x140
[ 4408.413821]  do_el0_svc+0x28/0x58
[ 4408.413911]  el0_svc+0x48/0x230
[ 4408.414035]  el0t_64_sync_handler+0xc0/0x108
[ 4408.414166]  el0t_64_sync+0x1b4/0x1b8

I tried to move this function out under the KVM MMU lock, but then I see
a debug entry is duplicated error. I am not sure where exactly this 
nested stage 2 debugfs entry create function should go, your help is
much appreciated.

Thanks,
Itaru.

>  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");
> +
> +	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
>