On Mon, Mar 25, 2024 at 6:35 PM Jonah Palmer <jonah.pal...@oracle.com> wrote: > > > > On 3/22/24 6:46 AM, Eugenio Perez Martin wrote: > > On Thu, Mar 21, 2024 at 4:57 PM Jonah Palmer <jonah.pal...@oracle.com> > > wrote: > >> > >> Implements in-order handling for most virtio devices using the > >> VIRTIO_F_IN_ORDER transport feature, specifically those who call > >> virtqueue_push to push their used elements onto the used ring. > >> > >> The logic behind this implementation is as follows: > >> > >> 1.) virtqueue_pop always enqueues VirtQueueElements in-order. > >> > >> virtqueue_pop always retrieves one or more buffer descriptors in-order > >> from the available ring and converts them into a VirtQueueElement. This > >> means that the order in which VirtQueueElements are enqueued are > >> in-order by default. > >> > >> By virtue, as VirtQueueElements are created, we can assign a sequential > >> key value to them. This preserves the order of buffers that have been > >> made available to the device by the driver. > >> > >> As VirtQueueElements are assigned a key value, the current sequence > >> number is incremented. > >> > >> 2.) Requests can be completed out-of-order. > >> > >> While most devices complete requests in the same order that they were > >> enqueued by default, some devices don't (e.g. virtio-blk). The goal of > >> this out-of-order handling is to reduce the impact of devices that > >> process elements in-order by default while also guaranteeing compliance > >> with the VIRTIO_F_IN_ORDER feature. > >> > >> Below is the logic behind handling completed requests (which may or may > >> not be in-order). > >> > >> 3.) Does the incoming used VirtQueueElement preserve the correct order? > >> > >> In other words, is the sequence number (key) assigned to the > >> VirtQueueElement the expected number that would preserve the original > >> order? > >> > >> 3a.) > >> If it does... immediately push the used element onto the used ring. > >> Then increment the next expected sequence number and check to see if > >> any previous out-of-order VirtQueueElements stored on the hash table > >> has a key that matches this next expected sequence number. > >> > >> For each VirtQueueElement found on the hash table with a matching key: > >> push the element on the used ring, remove the key-value pair from the > >> hash table, and then increment the next expected sequence number. Repeat > >> this process until we're unable to find an element with a matching key. > >> > >> Note that if the device uses batching (e.g. virtio-net), then we skip > >> the virtqueue_flush call and let the device call it themselves. > >> > >> 3b.) > >> If it does not... stash the VirtQueueElement, along with relevant data, > >> as a InOrderVQElement on the hash table. The key used is the order_key > >> that was assigned when the VirtQueueElement was created. > >> > >> Signed-off-by: Jonah Palmer <jonah.pal...@oracle.com> > >> --- > >> hw/virtio/virtio.c | 70 ++++++++++++++++++++++++++++++++++++-- > >> include/hw/virtio/virtio.h | 8 +++++ > >> 2 files changed, 76 insertions(+), 2 deletions(-) > >> > >> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c > >> index 40124545d6..40e4377f1e 100644 > >> --- a/hw/virtio/virtio.c > >> +++ b/hw/virtio/virtio.c > >> @@ -992,12 +992,56 @@ void virtqueue_flush(VirtQueue *vq, unsigned int > >> count) > >> } > >> } > >> > >> +void virtqueue_order_element(VirtQueue *vq, const VirtQueueElement *elem, > >> + unsigned int len, unsigned int idx, > >> + unsigned int count) > >> +{ > >> + InOrderVQElement *in_order_elem; > >> + > >> + if (elem->order_key == vq->current_order_idx) { > >> + /* Element is in-order, push to used ring */ > >> + virtqueue_fill(vq, elem, len, idx); > >> + > >> + /* Batching? Don't flush */ > >> + if (count) { > >> + virtqueue_flush(vq, count); > > > > The "count" parameter is the number of heads used, but here you're > > only using one head (elem). Same with the other virtqueue_flush in the > > function. > > > > True. This acts more as a flag than an actual count since, unless we're > batching (which in the current setup, the device would explicitly call > virtqueue_flush separately), this value will be either 0 or 1. >
If possible, I think it is better to keep consistent with the current API: fill+flush for batching, push for a single shot. > > Also, this function sometimes replaces virtqueue_fill and other > > replaces virtqueue_fill + virtqueue_flush (both examples in patch > > 6/8). I have the impression the series would be simpler if > > virtqueue_order_element is a static function just handling the > > virtio_vdev_has_feature(vq->vdev, VIRTIO_F_IN_ORDER) path of > > virtqueue_fill, so the caller does not need to know if the in_order > > feature is on or off. > > > > Originally I wanted this function to replace virtqueue_fill + > virtqueue_flush but after looking at virtio_net_receive_rcu and > vhost_svq_flush, where multiple virtqueue_fill's can be called before a > single virtqueue_flush, I added this 'if (count)' conditional to handle > both cases. > > I did consider virtqueue_order_element just handling the virtqueue_fill > path but then I wasn't sure how to handle calling virtqueue_flush when > retrieving out-of-order data from the hash table. > > For example, devices that call virtqueue_push would call virtqueue_fill > and then virtqueue_flush afterwards. In the scenario where, say, elem1 > was found out of order and put into the hash table, and then elem0 comes > along. For elem0 we'd call virtqueue_fill and then we should call > virtqueue_flush to keep the order going. Then we'd find elem1 and do the > same. I have trouble seeing how we could properly call virtqueue_flush > after finding out-of-order elements (that are now ready to be placed on > the used ring in-order) in the hash table. > I see, that's a good point indeed. The way I thought, it is a no-op in that case, and the later virtqueue_flush needs to check if it has pending buffers to use. The next question is what to do with the virtqueue_fill idx and virtqueue_flush count parameters. More on that in the cover letter, as the discussion goes that direction there. > >> + } > >> + > >> + /* Increment next expected order, search for more in-order > >> elements */ > >> + while ((in_order_elem = g_hash_table_lookup(vq->in_order_ht, > >> + GUINT_TO_POINTER(++vq->current_order_idx))) != > >> NULL) { > >> + /* Found in-order element, push to used ring */ > >> + virtqueue_fill(vq, in_order_elem->elem, in_order_elem->len, > >> + in_order_elem->idx); > >> + > >> + /* Batching? Don't flush */ > >> + if (count) { > >> + virtqueue_flush(vq, in_order_elem->count); > >> + } > >> + > >> + /* Remove key-value pair from hash table */ > >> + g_hash_table_remove(vq->in_order_ht, > >> + GUINT_TO_POINTER(vq->current_order_idx)); > >> + } > >> + } else { > >> + /* Element is out-of-order, stash in hash table */ > >> + in_order_elem = virtqueue_alloc_in_order_element(elem, len, idx, > >> + count); > >> + g_hash_table_insert(vq->in_order_ht, > >> GUINT_TO_POINTER(elem->order_key), > >> + in_order_elem); > >> + } > >> +} > >> + > >> void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem, > >> unsigned int len) > >> { > >> RCU_READ_LOCK_GUARD(); > >> - virtqueue_fill(vq, elem, len, 0); > >> - virtqueue_flush(vq, 1); > >> + if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_IN_ORDER)) { > >> + virtqueue_order_element(vq, elem, len, 0, 1); > >> + } else { > >> + virtqueue_fill(vq, elem, len, 0); > >> + virtqueue_flush(vq, 1); > >> + } > >> } > >> > >> /* Called within rcu_read_lock(). */ > >> @@ -1478,6 +1522,18 @@ void virtqueue_map(VirtIODevice *vdev, > >> VirtQueueElement *elem) > >> > >> false); > >> } > >> > >> +void *virtqueue_alloc_in_order_element(const VirtQueueElement *elem, > >> + unsigned int len, unsigned int idx, > >> + unsigned int count) > >> +{ > >> + InOrderVQElement *in_order_elem = g_malloc(sizeof(InOrderVQElement)); > >> + in_order_elem->elem = elem; > >> + in_order_elem->len = len; > >> + in_order_elem->idx = idx; > >> + in_order_elem->count = count; > >> + return in_order_elem; > >> +} > >> + > >> static void *virtqueue_alloc_element(size_t sz, unsigned out_num, > >> unsigned in_num) > >> { > >> VirtQueueElement *elem; > >> @@ -1626,6 +1682,11 @@ static void *virtqueue_split_pop(VirtQueue *vq, > >> size_t sz) > >> elem->in_sg[i] = iov[out_num + i]; > >> } > >> > >> + /* Assign key for in-order processing */ > >> + if (virtio_vdev_has_feature(vdev, VIRTIO_F_IN_ORDER)) { > >> + elem->order_key = vq->current_order_key++; > > > > Since you're adding this in both split_pop and packed_pop, why not add > > it in virtqueue_pop? > > > > I wanted to add this order_key to the VirtQueueElement after it was > created. I suppose I could do this directly in virtqueue_alloc_element > but I'd have to add another parameter to it, which might be unnecessary > given it'd only be applicable for this specific in_order feature. > > I also suppose I could just capture the VirtQueueElement being returned > from virtqueue_packed_pop/virtqueue_split_pop and make the assignment > there, but it felt out of place to do it in virtqueue_pop. > I see. I keep finding it simpler to do the assignment in one point only, as it is not a specific split / packed operation. But let's see. Thanks! > >> + } > >> + > >> vq->inuse++; > >> > >> trace_virtqueue_pop(vq, elem, elem->in_num, elem->out_num); > >> @@ -1762,6 +1823,11 @@ static void *virtqueue_packed_pop(VirtQueue *vq, > >> size_t sz) > >> vq->last_avail_wrap_counter ^= 1; > >> } > >> > >> + /* Assign key for in-order processing */ > >> + if (virtio_vdev_has_feature(vdev, VIRTIO_F_IN_ORDER)) { > >> + elem->order_key = vq->current_order_key++; > >> + } > >> + > >> vq->shadow_avail_idx = vq->last_avail_idx; > >> vq->shadow_avail_wrap_counter = vq->last_avail_wrap_counter; > >> > >> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h > >> index f83d7e1fee..eeeda397a9 100644 > >> --- a/include/hw/virtio/virtio.h > >> +++ b/include/hw/virtio/virtio.h > >> @@ -275,6 +275,14 @@ void virtio_del_queue(VirtIODevice *vdev, int n); > >> > >> void virtio_delete_queue(VirtQueue *vq); > >> > >> +void *virtqueue_alloc_in_order_element(const VirtQueueElement *elem, > >> + unsigned int len, unsigned int idx, > >> + unsigned int count); > >> + > >> +void virtqueue_order_element(VirtQueue *vq, const VirtQueueElement *elem, > >> + unsigned int len, unsigned int idx, > >> + unsigned int count); > >> + > >> void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem, > >> unsigned int len); > >> void virtqueue_flush(VirtQueue *vq, unsigned int count); > >> -- > >> 2.39.3 > >> > > >