Re: [PATCH v4 15/22] KVM: arm64: ITS: Sort the device and ITE lists

Marc Zyngier
2017-04-09T10:17:02Z

On Mon, Mar 27 2017 at 10:31:05 AM, Eric Auger <eric.auger@redhat.com> wrote:
> Natively sort the device and ITE lists in ascending
> deviceId/eventid order. This paves the way to optimized
> DTE and ITE scan in guest RAM table where entries are chained
> together using a next ID offset.
>
> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> Reviewed-by: Andre Przywara <andre.przywara@arm.com>
>
> ---
>
> v3 -> v4:
> - added Andre's R-b
> ---
> virt/kvm/arm/vgic/vgic-its.c | 36 ++++++++++++++++++++++++++++++++++--
> 1 file changed, 34 insertions(+), 2 deletions(-)
>
> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> index 2a1ccbf..7364b7d 100644
> --- a/virt/kvm/arm/vgic/vgic-its.c
> +++ b/virt/kvm/arm/vgic/vgic-its.c
> @@ -726,6 +726,21 @@ static void vgic_its_free_collection(struct vgic_its *its, u32 coll_id)
> kfree(collection);
> }
>
> +static void ite_list_insert_sorted(struct list_head *h, struct its_ite *ite)
> +{
> + struct list_head *pos = h->next;
> + u32 id = ite->event_id;
> +
> + while (pos != h) {
> + struct its_ite *iter =
> + list_entry(pos, struct its_ite, ite_list);
> + if (id < iter->event_id)
> + break;
> + pos = pos->next;
> + }
> + list_add_tail(&ite->ite_list, pos);
> +}
> +
> /* Must be called with its_lock mutex held */
> static int vgic_its_alloc_ite(struct its_device *device,
> struct its_ite **itep,
> @@ -742,7 +757,7 @@ static int vgic_its_alloc_ite(struct its_device *device,
> ite->collection = collection;
> ite->lpi = lpi_id;
>
> - list_add_tail(&ite->ite_list, &device->itt_head);
> + ite_list_insert_sorted(&device->itt_head, ite);
> *itep = ite;
> return 0;
> }
> @@ -835,6 +850,22 @@ static void vgic_its_unmap_device(struct kvm *kvm, struct its_device *device)
> kfree(device);
> }
>
> +static void device_list_insert_sorted(struct list_head *h,
> + struct its_device *dev)
> +{
> + struct list_head *pos = h->next;
> + u32 id = dev->device_id;
> +
> + while (pos != h) {
> + struct its_device *iter =
> + list_entry(pos, struct its_device, dev_list);
> + if (id < iter->device_id)
> + break;
> + pos = pos->next;
> + }
> + list_add_tail(&dev->dev_list, pos);
> +}
> +
> /* Must be called with its_lock mutex held */
> static int vgic_its_alloc_device(struct vgic_its *its,
> struct its_device **devp,
> @@ -852,7 +883,8 @@ static int vgic_its_alloc_device(struct vgic_its *its,
> device->nb_eventid_bits = nb_eventid_bits;
> INIT_LIST_HEAD(&device->itt_head);
>
> - list_add_tail(&device->dev_list, &its->device_list);
> + device_list_insert_sorted(&its->device_list, device);
> +
> *devp = device;
>
> return 0;

What is the actual gain for sorting the list at runtime, vs sorting it
at save time? A save/restore operation is a very rare event compared to
the normal use of the ITS, so I'd rather put the overhead on the rarest
event if possible.

Thanks,

M.
--
Jazz is not dead. It just smells funny.