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

Sean Christopherson
2019-10-24T19:38:56Z

On Tue, Oct 22, 2019 at 05:53:27PM +0200, Paolo Bonzini wrote:
> On 22/10/19 17:52, Sean Christopherson wrote:
> >
> > Anyways, I'm not at all opposed to adding comments, just want to make sure
> > I'm not forgetting something. If it's ok with you, I'll comment the code
> > and/or functions and reply here to refine them without having to respin
> > the whole series.
>
> Yes, I agree this is better.

Here's what I ended up with. I also added kvm_memslot_insert_back() to
help document the purpose of incrementing used_slots, and renamed
kvm_shift_memslots_forward()->kvm_memslot_move_backward() and
kvm_shift_memslots_backward()->kvm_memslot_move_forward() because I was
having trouble reconciling having the comments focus on the changed
memslot while the names of the functions reflected what happens to the
other memslots.

/*
* Delete a memslot by decrementing the number of used slots and shifting all
* other entries in the array forward one spot.
*/
static inline void kvm_memslot_delete(struct kvm_memslots *slots,
struct kvm_memory_slot *memslot)
{
struct kvm_memory_slot *mslots = slots->memslots;
int i;

if (WARN_ON(slots->id_to_index[memslot->id] = -1))
return;

slots->used_slots--;

for (i = slots->id_to_index[memslot->id]; i < slots->used_slots; i++) {
mslots[i] = mslots[i + 1];
slots->id_to_index[mslots[i].id] = i;
}
mslots[i] = *memslot;
slots->id_to_index[memslot->id] = -1;
}

/*
* "Insert" a new memslot by incrementing the number of used slots. Returns
* the new slot's initial index into the memslots array.
*/
static inline int kvm_memslot_insert_back(struct kvm_memslots *slots)
{
return slots->used_slots++;
}

/*
* Move a changed memslot backwards in the array by shifting existing slots
* with a higher GFN toward the front 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 update. 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. Returns the changed memslot's final index
* into the memslots array.
*/
static inline int kvm_memslot_move_forward(struct kvm_memslots *slots,
struct kvm_memory_slot *memslot,
int start)
{
struct kvm_memory_slot *mslots = slots->memslots;
int i;

for (i = start; i > 0; 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 back one and update its index. */
mslots[i] = mslots[i - 1];
slots->id_to_index[mslots[i].id] = i;
}
return i;
}

/*
* Re-sort memslots based on their GFN to account for an added, deleted, or
* moved memslot. Sorting memslots allows using a binary search during memslot
* lookup.
*
* IMPORTANT: Slots are sorted from highest GFN to lowest GFN! I.e. the entry
* at memslots[0] has the highest GFN.
*
* The sorting algorithm takes advantage of having initially sorted memslots
* and knowing the position of the changed memslot. Sorting is also optimized
* by not swapping the updated memslot and instead only shifting other memslots
* and tracking the new index for the update memslot. Only once its final
* index is known is the updated memslot copied into its position in the array.
*
* - When deleting a memslot, the deleted memslot simply needs to be moved to
* the end of the array.
*
* - When creating a memslot, the algorithm "inserts" the new memslot at the
* end of the array and then it forward to its correct location.
*
* - When moving a memslot, the algorithm first moves the updated memslot
* backward to handle the scenario where the memslot's GFN was changed to a
* lower value. update_memslots() then falls through and runs the same flow
* as creating a memslot to move the memslot forward to handle the scenario
* where its GFN was changed to a higher value.
*
* Note, slots are sorted from highest->lowest instead of lowest->highest for
* historical reasons. Originally, invalid memslots where denoted by having
* GFN=0, thus sorting from highest->lowest naturally sorted invalid memslots
* to the end of the array. The current algorithm uses dedicated logic when
* deleting a memslot and thus does not rely on invalid memslots having GFN=0.
*/
static void update_memslots(struct kvm_memslots *slots,
struct kvm_memory_slot *memslot,
enum kvm_mr_change change)
{
int i;

if (change = KVM_MR_DELETE) {
kvm_memslot_delete(slots, memslot);
} else {
if (change = KVM_MR_CREATE)
i = kvm_memslot_insert_back(slots);
else
i = kvm_memslot_move_backward(slots, memslot);
i = kvm_memslot_move_forward(slots, memslot, i);

/*
* Copy the memslot to its new position in memslots and update
* its index accordingly.
*/
slots->memslots[i] = *memslot;
slots->id_to_index[memslot->id] = i;
}
}