Re: [PATCH v1 12/13] KVM: x86/mmu: Add tracepoint for splitting huge pages

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 13, 2021, David Matlack wrote:
> Add a tracepoint that records whenever KVM eagerly splits a huge page.
> 
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  arch/x86/kvm/mmu/mmutrace.h | 20 ++++++++++++++++++++
>  arch/x86/kvm/mmu/tdp_mmu.c  |  2 ++
>  2 files changed, 22 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmutrace.h b/arch/x86/kvm/mmu/mmutrace.h
> index de5e8e4e1aa7..4feabf773387 100644
> --- a/arch/x86/kvm/mmu/mmutrace.h
> +++ b/arch/x86/kvm/mmu/mmutrace.h
> @@ -416,6 +416,26 @@ TRACE_EVENT(
>  	)
>  );
>  
> +TRACE_EVENT(
> +	kvm_mmu_split_huge_page,
> +	TP_PROTO(u64 gfn, u64 spte, int level),
> +	TP_ARGS(gfn, spte, level),
> +
> +	TP_STRUCT__entry(
> +		__field(u64, gfn)
> +		__field(u64, spte)
> +		__field(int, level)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->gfn = gfn;
> +		__entry->spte = spte;
> +		__entry->level = level;
> +	),
> +
> +	TP_printk("gfn %llx spte %llx level %d", __entry->gfn, __entry->spte, __entry->level)
> +);
> +
>  #endif /* _TRACE_KVMMMU_H */
>  
>  #undef TRACE_INCLUDE_PATH
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index be5eb74ac053..e6910b9b5c12 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1325,6 +1325,8 @@ tdp_mmu_split_huge_page_atomic(struct kvm *kvm, struct tdp_iter *iter, struct kv
>  	u64 child_spte;
>  	int i;
>  
> +	trace_kvm_mmu_split_huge_page(iter->gfn, huge_spte, level);

This should either be called iff splitting is successful, or it should record
whether or not the split was successful.  The latter is probably useful info,
and easy to do, e.g. assuming this is changed to return an int like the lower
helpers:


	ret = tdp_mmu_install_sp_atomic(kvm, iter, sp, false);

	/*
	 * tdp_mmu_install_sp_atomic will handle subtracting the split huge
	 * page from stats, but we have to manually update the new present child
	 * pages on success.
	 */
	if (!ret)
		kvm_update_page_stats(kvm, level - 1, PT64_ENT_PER_PAGE);

	trace_kvm_mmu_split_huge_page(iter->gfn, huge_spte, level, ret);

	return ret;

and then the tracpoint can do 'ret ? "failed" : "succeeded"' or something.

> +
>  	init_child_tdp_mmu_page(sp, iter);
>  
>  	for (i = 0; i < PT64_ENT_PER_PAGE; i++) {
> -- 
> 2.34.1.173.g76aa8bc2d0-goog
>