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Message-ID: <Z4YcmXQEt99FXIid@yzhao56-desk.sh.intel.com>
Date: Tue, 14 Jan 2025 16:13:13 +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>
Subject: Re: [PATCH 4/5] KVM: Check for empty mask of harvested dirty ring
entries in caller
On Mon, Jan 13, 2025 at 08:48:28AM -0800, Sean Christopherson wrote:
> 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.
> */
Nice log.
> 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.
> */
LGTM!
> > > + */
> > > + 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
> > >
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