Re: [PATCH 04/24] kvm: x86/mmu: change TDP MMU yield function returns to match cond_resched

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 12, 2021, Ben Gardon wrote:
> Currently the TDP MMU yield / cond_resched functions either return
> nothing or return true if the TLBs were not flushed. These are confusing
> semantics, especially when making control flow decisions in calling
> functions.
> 
> To clean things up, change both functions to have the same
> return value semantics as cond_resched: true if the thread yielded,
> false if it did not. If the function yielded in the _flush_ version,
> then the TLBs will have been flushed.
> 
> Reviewed-by: Peter Feiner <pfeiner@google.com>
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 38 +++++++++++++++++++++++++++++---------
>  1 file changed, 29 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 2ef8615f9dba..b2784514ca2d 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -413,8 +413,15 @@ static inline void tdp_mmu_set_spte_no_dirty_log(struct kvm *kvm,
>  			 _mmu->shadow_root_level, _start, _end)
>  
>  /*
> - * Flush the TLB if the process should drop kvm->mmu_lock.
> - * Return whether the caller still needs to flush the tlb.
> + * Flush the TLB and yield if the MMU lock is contended or this thread needs to
> + * return control to the scheduler.
> + *
> + * If this function yields, it will also reset the tdp_iter's walk over the
> + * paging structure and the calling function should allow the iterator to
> + * continue its traversal from the paging structure root.
> + *
> + * Return true if this function yielded, the TLBs were flushed, and the
> + * iterator's traversal was reset. Return false if a yield was not needed.
>   */
>  static bool tdp_mmu_iter_flush_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>  {
> @@ -422,18 +429,30 @@ static bool tdp_mmu_iter_flush_cond_resched(struct kvm *kvm, struct tdp_iter *it
>  		kvm_flush_remote_tlbs(kvm);
>  		cond_resched_lock(&kvm->mmu_lock);
>  		tdp_iter_refresh_walk(iter);
> -		return false;
> -	} else {
>  		return true;
> -	}
> +	} else
> +		return false;

Kernel style is to have curly braces on all branches if any branch has 'em.  Or,
omit the else since the taken branch always returns.  I think I prefer the latter?

>  }
>  
> -static void tdp_mmu_iter_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
> +/*
> + * Yield if the MMU lock is contended or this thread needs to return control
> + * to the scheduler.
> + *
> + * If this function yields, it will also reset the tdp_iter's walk over the
> + * paging structure and the calling function should allow the iterator to
> + * continue its traversal from the paging structure root.
> + *
> + * Return true if this function yielded and the iterator's traversal was reset.
> + * Return false if a yield was not needed.
> + */
> +static bool tdp_mmu_iter_cond_resched(struct kvm *kvm, struct tdp_iter *iter)
>  {
>  	if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
>  		cond_resched_lock(&kvm->mmu_lock);
>  		tdp_iter_refresh_walk(iter);
> -	}
> +		return true;
> +	} else
> +		return false;

Same here.

>  }
>  
>  /*
> @@ -470,7 +489,8 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
>  		tdp_mmu_set_spte(kvm, &iter, 0);
>  
>  		if (can_yield)
> -			flush_needed = tdp_mmu_iter_flush_cond_resched(kvm, &iter);
> +			flush_needed = !tdp_mmu_iter_flush_cond_resched(kvm,
> +									&iter);

As with the existing code, I'd let this poke out.  Alternatively, this could be
written as:

		flush_needed = !can_yield ||
			       !tdp_mmu_iter_flush_cond_resched(kvm, &iter);

>  		else
>  			flush_needed = true;
>  	}
> @@ -1072,7 +1092,7 @@ static void zap_collapsible_spte_range(struct kvm *kvm,
>  
>  		tdp_mmu_set_spte(kvm, &iter, 0);
>  
> -		spte_set = tdp_mmu_iter_flush_cond_resched(kvm, &iter);
> +		spte_set = !tdp_mmu_iter_flush_cond_resched(kvm, &iter);
>  	}
>  
>  	if (spte_set)
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
>