Re: [PATCH v2 27/28] KVM: arm64: Add tracepoints for S1 page table walk

[Leonardo Bras <leo.bras@arm.com>]

On Fri, Oct 23, 2026 at 01:56:09PM -0700, Oliver Upton wrote:
> MMU walk steps are a useful tidbit of information for KVM hackers. As
> these events are far closer to modelling CPU architecture than exposing
> KVM internals it clears the bar for adding tracepoints.
> 
> Signed-off-by: Oliver Upton <oupton@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_nested.h |  8 +++++
>  arch/arm64/kvm/at.c                 | 18 +++++-----
>  arch/arm64/kvm/trace_arm.h          | 52 +++++++++++++++++++++++++++++
>  3 files changed, 68 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> index 81d8b1676fe4..341604b619cc 100644
> --- a/arch/arm64/include/asm/kvm_nested.h
> +++ b/arch/arm64/include/asm/kvm_nested.h
> @@ -343,6 +343,14 @@ struct s1_walk_info {
>  	bool			haft;
>  };
>  
> +struct s1_walk_step {
> +	u64			desc;
> +	u64			desc_ipa;
> +	u64			desc_pa;
> +	struct kvm_s2_trans	s2_trans;
> +	int			level;
> +};
> +
>  struct s1_walk_result {
>  	union {
>  		struct {
> diff --git a/arch/arm64/kvm/at.c b/arch/arm64/kvm/at.c
> index 8f1be3af64e8..1a49eb3c288a 100644
> --- a/arch/arm64/kvm/at.c
> +++ b/arch/arm64/kvm/at.c
> @@ -11,6 +11,8 @@
>  #include <asm/kvm_mmu.h>
>  #include <asm/lsui.h>
>  
> +#include "trace.h"
> +
>  static int get_ia_size(struct s1_walk_info *wi)
>  {
>  	return 64 - wi->txsz;
> @@ -433,14 +435,6 @@ static int setup_s1_walk(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
>  	return -EFAULT;
>  }
>  
> -struct s1_walk_step {
> -	u64			desc;
> -	u64			desc_ipa;
> -	u64			desc_pa;
> -	struct kvm_s2_trans	s2_trans;
> -	int			level;
> -};
> -
>  static int kvm_read_s1_desc(struct kvm_vcpu *vcpu, u64 pa, u64 *desc,
>  			    struct s1_walk_info *wi)
>  {
> @@ -492,10 +486,10 @@ static int handle_desc_update(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
>  			      struct s1_walk_step *ws, struct s1_walk_result *wr,
>  			      struct kvm_walk_access *access)
>  {
> -	u64 old, new;
> +	u64 old, old_cpu, new;
>  	int ret;
>  
> -	old = new = ws->desc;
> +	old = old_cpu = new = ws->desc;
>  
>  	if (should_set_access_flag(wi, ws, access))
>  		new |= PTE_AF;
> @@ -529,6 +523,8 @@ static int handle_desc_update(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
>  		return ret;
>  	}
>  
> +	trace_kvm_s1_desc_update(ws, access->ia, old_cpu);
> +
>  	wr->desc = ws->desc;
>  	wr->dirty = !(wr->desc & PTE_RDONLY);
>  	return 0;
> @@ -614,6 +610,8 @@ static int walk_s1(struct kvm_vcpu *vcpu, struct s1_walk_info *wi,
>  			return ret;
>  		}
>  
> +		trace_kvm_s1_walk_step(&ws, access->ia);
> +
>  		/* Invalid descriptor */
>  		if (!(ws.desc & BIT(0)))
>  			goto transfault;
> diff --git a/arch/arm64/kvm/trace_arm.h b/arch/arm64/kvm/trace_arm.h
> index 9c60f6465c78..9fe9b57f6fa8 100644
> --- a/arch/arm64/kvm/trace_arm.h
> +++ b/arch/arm64/kvm/trace_arm.h
> @@ -415,6 +415,58 @@ TRACE_EVENT(kvm_forward_sysreg_trap,
>  		      sys_reg_Op2(__entry->sysreg))
>  );
>  
> +TRACE_EVENT(kvm_s1_walk_step,
> +	    TP_PROTO(struct s1_walk_step *ws, u64 ia),
> +	    TP_ARGS(ws, ia),
> +
> +	    TP_STRUCT__entry(
> +		__field(u64,	ia)
> +		__field(u64,	desc)
> +		__field(u64,	ipa)
> +		__field(u64,	pa)
> +		__field(int,	level)
> +	    ),
> +
> +	    TP_fast_assign(
> +		__entry->ia	= ia;
> +		__entry->desc	= ws->desc;
> +		__entry->ipa	= ws->desc_ipa;
> +		__entry->pa	= ws->desc_pa;
> +		__entry->level	= ws->level;
> +	    ),
> +
> +	    TP_printk("ia: 0x%llx desc: 0x%llx ipa: 0x%llx pa: 0x%llx level: %d",
> +		      __entry->ia, __entry->desc, __entry->ipa, __entry->pa,
> +		      __entry->level)
> +);
> +
> +TRACE_EVENT(kvm_s1_desc_update,
> +	    TP_PROTO(struct s1_walk_step *ws, u64 ia, u64 old),
> +	    TP_ARGS(ws, ia, old),
> +
> +	    TP_STRUCT__entry(
> +		__field(u64,	ia)
> +		__field(u64,	new)
> +		__field(u64,	old)
> +		__field(u64,	ipa)
> +		__field(u64,	pa)
> +		__field(int,	level)
> +	    ),
> +
> +	    TP_fast_assign(
> +		__entry->ia	= ia;
> +		__entry->new	= ws->desc;
> +		__entry->old	= old;
> +		__entry->ipa	= ws->desc_ipa;
> +		__entry->pa	= ws->desc_pa;
> +		__entry->level	= ws->level;
> +	    ),
> +
> +	    TP_printk("ia: 0x%llx new: 0x%llx old: 0x%llx ipa: 0x%llx pa: 0x%llx level: %d",
> +		      __entry->ia, __entry->new, __entry->old, __entry->ipa,
> +		      __entry->pa, __entry->level)
> +);
> +
>  #endif /* _TRACE_ARM_ARM64_KVM_H */
>  
>  #undef TRACE_INCLUDE_PATH
> -- 
> 2.47.3
> 


Tracepoints :)

FWIW:
Reviewed-by: Leonardo Bras <leo.bras@arm.com>

Thanks!
Leo