Subject: Re: [PATCH v2 14/15] KVM: Terminate memslot walks via used_slots

From: Sean Christopherson <[email-masked]>
Date: Thu, 24 Oct 2019 13:48:26 -0700

On Thu, Oct 24, 2019 at 10:24:09PM +0200, Paolo Bonzini wrote:
> On 24/10/19 21:38, Sean Christopherson wrote:
> > only
> >  * its new index into the array is update.
> 
> s/update/tracked/?

Ya, tracked is better.  Waffled between updated and tracked, chose poorly :-)

>   Returns the changed memslot's
> >  * current index into the memslots array.
> >  */
> > static inline int kvm_memslot_move_backward(struct kvm_memslots *slots,
> > 					    struct kvm_memory_slot *memslot)
> > {
> > 	struct kvm_memory_slot *mslots = slots->memslots;
> > 	int i;
> > 
> > 	if (WARN_ON_ONCE(slots->id_to_index[memslot->id] == -1) ||
> > 	    WARN_ON_ONCE(!slots->used_slots))
> > 		return -1;
> > 
> > 	for (i = slots->id_to_index[memslot->id]; i < slots->used_slots - 1; i++) {
> > 		if (memslot->base_gfn > mslots[i + 1].base_gfn)
> > 			break;
> > 
> > 		WARN_ON_ONCE(memslot->base_gfn == mslots[i + 1].base_gfn);
> > 
> > 		/* Shift the next memslot forward one and update its index. */
> > 		mslots[i] = mslots[i + 1];
> > 		slots->id_to_index[mslots[i].id] = i;
> > 	}
> > 	return i;
> > }
> > 
> > /*
> >  * Move a changed memslot forwards in the array by shifting existing slots with
> >  * a lower GFN toward the back of the array.  Note, the changed memslot itself
> >  * is not preserved in the array, i.e. not swapped at this time, only its new
> >  * index into the array is updated
> 
> Same here?
> 
> >  * Note, slots are sorted from highest->lowest instead of lowest->highest for
> >  * historical reasons.
> 
> Not just that, the largest slot (with all RAM above 4GB) is also often
> at the highest address at least on x86.

Ah, increasing the odds of a quick hit on lookup...but only when using a
linear search.  The binary search starts in the middle, so that
optimization is also historical :-)

> But we could sort them by size now, so I agree to call these historical
> reasons.

That wouldn't work with the binary search though.

> The code itself is fine, thanks for the work on documenting it.
> 
> Paolo
> 

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