linux-kernel - Re: [PATCH 4/5] KVM: Check for empty mask of harvested dirty ring entries in caller

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Message-ID: <Z4VD3AaQskK7IkYU@google.com>
Date: Mon, 13 Jan 2025 08:48:28 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.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>
Subject: Re: [PATCH 4/5] KVM: Check for empty mask of harvested dirty ring
 entries in caller

On Mon, Jan 13, 2025, Yan Zhao wrote:
> On Fri, Jan 10, 2025 at 05:04:08PM -0800, Sean Christopherson wrote:
> > @@ -163,14 +157,31 @@ 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.
> > +		 */
> I really like the logs as it took me quite a while figuring out how this part of
> the code works :)
> 
> Does "processed" mean the entries have been reset, and "gathered" means they've
> been read from the ring?

Yeah.

> I'm not sure, but do you like this version? e.g.
> "Combined reset of the harvested entries that can be identified by curr_slot
> plus cur_offset+mask" ?

I have no objection to documenting the mechanics *and* the high level intent,
but I definitely want to document the "what", not just the "how".

> > +		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.
> > +		 */
> What about
> "Save the current slot and cur_offset (with mask initialized to 1) to check if
> any future entries can be found for a combined reset." ?

Hmm, what if I add a comment at the top to document the overall behavior and the
variables,

	/*
	 * 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
	 * 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.
	 */
	u32 cur_slot, next_slot;
	u64 cur_offset, next_offset;
	unsigned long mask = 0;
	struct kvm_dirty_gfn *entry;

and then keep this as:

		/*
		 * The current slot was reset or this is the first harvested
		 * entry, (re)initialize the batching 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. The loop only performs a reset when an entry can't
> > +	 * be coalesced, i.e. always leaves at least one entry pending.
> The loop only performs a reset when an entry can be coalesced?

No, if an entry can be coalesced then the loop doesn't perform a reset.  Does
this read better?

	/*
	 * 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 "current" the entry(s), and that reset processes the
	 * _current_ entry(s), i.e. the last harvested entry, a.k.a. next, will
	 * 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
> > -- 
> > 2.47.1.613.gc27f4b7a9f-goog
> >