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| From: | Jonah Palmer |
| Subject: | Re: [RFC 0/8] virtio,vhost: Add VIRTIO_F_IN_ORDER support |
| Date: | Tue, 26 Mar 2024 12:49:15 -0400 |
| User-agent: | Mozilla Thunderbird |
On 3/25/24 4:33 PM, Eugenio Perez Martin wrote:
On Mon, Mar 25, 2024 at 5:52 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:On 3/22/24 7:18 AM, Eugenio Perez Martin wrote:On Thu, Mar 21, 2024 at 4:57 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:The goal of these patches is to add support to a variety of virtio and vhost devices for the VIRTIO_F_IN_ORDER transport feature. This feature indicates that all buffers are used by the device in the same order in which they were made available by the driver. These patches attempt to implement a generalized, non-device-specific solution to support this feature. The core feature behind this solution is a buffer mechanism in the form of GLib's GHashTable. The decision behind using a hash table was to leverage their ability for quick lookup, insertion, and removal operations. Given that our keys are simply numbers of an ordered sequence, a hash table seemed like the best choice for a buffer mechanism. --------------------- The strategy behind this implementation is as follows: We know that buffers that are popped from the available ring and enqueued for further processing will always done in the same order in which they were made available by the driver. Given this, we can note their order by assigning the resulting VirtQueueElement a key. This key is a number in a sequence that represents the order in which they were popped from the available ring, relative to the other VirtQueueElements. For example, given 3 "elements" that were popped from the available ring, we assign a key value to them which represents their order (elem0 is popped first, then elem1, then lastly elem2): elem2 -- elem1 -- elem0 ---> Enqueue for processing (key: 2) (key: 1) (key: 0) Then these elements are enqueued for further processing by the host. While most devices will return these completed elements in the same order in which they were enqueued, some devices may not (e.g. virtio-blk). To guarantee that these elements are put on the used ring in the same order in which they were enqueued, we can use a buffering mechanism that keeps track of the next expected sequence number of an element. In other words, if the completed element does not have a key value that matches the next expected sequence number, then we know this element is not in-order and we must stash it away in a hash table until an order can be made. The element's key value is used as the key for placing it in the hash table. If the completed element has a key value that matches the next expected sequence number, then we know this element is in-order and we can push it on the used ring. Then we increment the next expected sequence number and check if the hash table contains an element at this key location. If so, we retrieve this element, push it to the used ring, delete the key-value pair from the hash table, increment the next expected sequence number, and check the hash table again for an element at this new key location. This process is repeated until we're unable to find an element in the hash table to continue the order. So, for example, say the 3 elements we enqueued were completed in the following order: elem1, elem2, elem0. The next expected sequence number is 0: exp-seq-num = 0: elem1 --> elem1.key == exp-seq-num ? --> No, stash it (key: 1) | | v ================ |key: 1 - elem1| ================ --------------------- exp-seq-num = 0: elem2 --> elem2.key == exp-seq-num ? --> No, stash it (key: 2) | | v ================ |key: 1 - elem1| |--------------| |key: 2 - elem2| ================ --------------------- exp-seq-num = 0: elem0 --> elem0.key == exp-seq-num ? --> Yes, push to used ring (key: 0) exp-seq-num = 1: lookup(table, exp-seq-num) != NULL ? --> Yes, push to used ring, remove elem from table | v ================ |key: 2 - elem2| ================ exp-seq-num = 2: lookup(table, exp-seq-num) != NULL ? --> Yes, push to used ring, remove elem from table | v ================ | *empty* | ================ exp-seq-num = 3: lookup(table, exp-seq-num) != NULL ? --> No, done ---------------------I think to use a hashtable to handle this has an important drawback: it hurts performance on the devices that are using right in-order because of hash calculus, to benefit devices that are using it badly by using descriptors out of order. We should use data structs that are as free as possible for the first, and we don't care to worse the experience of the devices that enable in_order and they shouldn't.Right, because if descriptors are coming in in-order, we still search the (empty) hash table. Hmm... what if we introduced a flag to see if we actually should bother searching the hash table? That way we avoid the cost of searching when we really don't need to.So I suggest reusing vq->used_elems array vq. At each used descriptor written in the used ring, you know the next head is elem->index + elem->ndescs, so you can check if that element has been filled or not. If used, it needs to be flushed too. If not used, just return. Of course virtqueue_flush also needs to take this into account. What do you think, does it make sense to you?I'm having a bit of trouble understanding the suggestion here. Would you mind elaborating a bit more for me on this? For example, say elem0, elem1, and elem2 were enqueued in-order (elem0 being first, elem2 last) and then elem2 finishes first, elem1 second, and elem0 third. Given that these elements finish out-of-order, how would you handle these out-of-order elements using your suggestion?virtqueue_fill is called first with elem2. So vq->used_elems[2 % vq->num] is filled with the needed information of the descriptor: index, len and ndescs. idx function parameter is ignored. Optionally, virtqueue_push is called. It checks if vq->used_elems[vq->used_idx] is valid. valid can be elem->in_num + elem->out_num > 0, and reset them on every used ring write. If it is not valid, this is a no-op. Currently, it is not valid. Same process for elem1. virtqueue_fill is the same for elem0. But now virtqueue_flush gets interesting, as it detects vq->used_elems[0] is used. It scans for the first not-used element, and it finds it is vq->used_elems[3]. So it needs to write an used elem with id = 2 and the corresponding length. Maybe it is interesting to implement ways to improve the look for the last used descriptor, but if any I'd go for a bitmap and always on top of the basis series. The algorithm has not been tested, so maybe I've missed something. Thanks!
Thank you for taking the time to clarify for this for me, I appreciate it.I spent some time yesterday and this morning working this over in my head. I believe I understand what you're trying to do here and it makes more sense than employing a data structure like a hash table for this kind of job. However, I have a few questions regarding this implementation.
So, one question is on the reuse of the VirtQueue's used_elems array. Wont reusing this array cause issues with packed VQ operations, since it also uses this array? If we want to stick with using this array specifically, perhaps we may need to rewrite its logic if the device has negotiated the in_order feature? E.g.
virtqueue_packed_flush (...) {
if (virtio_vdev_has_feature(vdev, VIRTIO_F_IN_ORDER) {
// new logic
} else {
// current logic
}
}
-----------
Regarding this paragraph:
"virtqueue_fill is called first with elem2. So vq->used_elems[2 %
vq->num] is filled with the needed information of the descriptor:
index, len and ndescs. idx function parameter is ignored."
This looks exactly like virtqueue_packed_fill except for the idx
parameter we'd pass in (sequence_num % vq->vring.num).
In any case, regardless of whether this element being passed in is considered to be in-order or not, we still add this element to vq->used_elems in virtqueue_fill. Ok, got it.
Then you say "Optionally, virtqueue_push is called". I assume by "optionally" you mean we need to know if this is a single-shot operation or a batched operation. A single-shot operation would call for virtqueue_push whereas a batched operation would just use virtqueue_fill. If this is what you meant by that then ok, I understand that too.
However, I think before we start considering whether or not we need to call virtqueue_push or continue with virtqueue_fill, we first should know whether or not this element is in-order. And I think to do that we should use the check you mentioned:
if (vq->used_elems[vq->used_idx].in_num + vq->used_elems[vq->used_idx].out_num > 0)
or perhaps: if (vq->used_elems[vq->used_idx] != NULL)If the element is found not to be in-order, I assume we return and we are done with the handling of this element for now.
Now my confusion with this part comes from calling virtqueue_push inside of the virtqueue_fill function. Wouldn't calling virtqueue_push inside of virtqueue_fill present some kind of recursive execution path? Unless I'm missing something here, this probably isn't something we need to do, right?
-----------Lastly, when execution reaches virtqueue_flush, what would define an element as unused? Perhaps...
if (vq->used_elems[i] == NULL) or if (vq->used_elems[i].in_num + vq->used_elems[i].out_num > 0) Thanks Eugenio!
Thanks :)Thanks!Jonah Palmer (8): virtio: Define InOrderVQElement virtio: Create/destroy/reset VirtQueue In-Order hash table virtio: Define order variables virtio: Implement in-order handling for virtio devices virtio-net: in-order handling vhost-svq: in-order handling vhost/vhost-user: Add VIRTIO_F_IN_ORDER to vhost feature bits virtio: Add VIRTIO_F_IN_ORDER property definition hw/block/vhost-user-blk.c | 1 + hw/net/vhost_net.c | 2 + hw/net/virtio-net.c | 6 +- hw/scsi/vhost-scsi.c | 1 + hw/scsi/vhost-user-scsi.c | 1 + hw/virtio/vhost-shadow-virtqueue.c | 15 ++++- hw/virtio/vhost-user-fs.c | 1 + hw/virtio/vhost-user-vsock.c | 1 + hw/virtio/virtio.c | 103 ++++++++++++++++++++++++++++- include/hw/virtio/virtio.h | 20 +++++- net/vhost-vdpa.c | 1 + 11 files changed, 145 insertions(+), 7 deletions(-) -- 2.39.3
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