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

Sean Christopherson
2019-10-24T20:48:26Z

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
>