linux-kernel - Re: [PATCH v3 4/6] KVM: Check for empty mask of harvested dirty ring entries in caller

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Message-ID: <aC2Z2U/HR5wAay3s@yzhao56-desk.sh.intel.com>
Date: Wed, 21 May 2025 17:16:09 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Peter Xu <peterx@...hat.com>, Maxim Levitsky
	<mlevitsk@...hat.com>, Binbin Wu <binbin.wu@...ux.intel.com>, James Houghton
	<jthoughton@...gle.com>, Pankaj Gupta <pankaj.gupta@....com>
Subject: Re: [PATCH v3 4/6] KVM: Check for empty mask of harvested dirty ring
 entries in caller

On Fri, May 16, 2025 at 02:35:38PM -0700, Sean Christopherson wrote:
> @@ -108,15 +105,24 @@ static inline bool kvm_dirty_gfn_harvested(struct kvm_dirty_gfn *gfn)
>  int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
>  			 int *nr_entries_reset)
>  {
> +	/*
> +	 * To minimize mmu_lock contention, batch resets for harvested entries
> +	 * whose gfns are in the same slot, and are within N frame numbers of
> +	 * each other, where N is the number of bits in an unsigned long.  For
Suppose N is 64,

> +	 * simplicity, process the current set of entries when the next entry
> +	 * can't be included in the batch.
> +	 *
> +	 * Track the current batch slot, the gfn offset into the slot for the
> +	 * batch, and the bitmask of gfns that need to be reset (relative to
> +	 * offset).  Note, the offset may be adjusted backwards, e.g. so that
> +	 * a sequence of gfns X, X-1, ... X-N can be batched.
X-N can't be batched, right?

> +	 */
>  	u32 cur_slot, next_slot;
>  	u64 cur_offset, next_offset;
> -	unsigned long mask;
> +	unsigned long mask = 0;
>  	struct kvm_dirty_gfn *entry;
>  	bool first_round = true;
>  
> -	/* This is only needed to make compilers happy */
> -	cur_slot = cur_offset = mask = 0;
> -
>  	while (likely((*nr_entries_reset) < INT_MAX)) {
>  		if (signal_pending(current))
>  			return -EINTR;
> @@ -164,14 +170,34 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring,
>  				continue;
>  			}
>  		}
> -		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> +
> +		/*
> +		 * Reset the slot for all the harvested entries that have been
> +		 * gathered, but not yet fully processed.
> +		 */
> +		if (mask)
> +			kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> +
> +		/*
> +		 * The current slot was reset or this is the first harvested
> +		 * entry, (re)initialize the metadata.
> +		 */
>  		cur_slot = next_slot;
>  		cur_offset = next_offset;
>  		mask = 1;
>  		first_round = false;
>  	}
>  
> -	kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> +	/*
> +	 * Perform a final reset if there are harvested entries that haven't
> +	 * been processed, which is guaranteed if at least one harvested was
> +	 * found.  The loop only performs a reset when the "next" entry can't
> +	 * be batched with the "current" entry(s), and that reset processes the
> +	 * _current_ entry(s); i.e. the last harvested entry, a.k.a. next, will
> +	 * always be left pending.
> +	 */
> +	if (mask)
> +		kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>  
>  	/*
>  	 * The request KVM_REQ_DIRTY_RING_SOFT_FULL will be cleared
> -- 
> 2.49.0.1112.g889b7c5bd8-goog
>