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: <aC3pWOUnMWvsxYWc@google.com>
Date: Wed, 21 May 2025 07:55:20 -0700
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>, 
	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 Wed, May 21, 2025, Yan Zhao wrote:
> 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?

Hah!  Yeah, off-by-one error.