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Re: [RFC 0/8] virtio,vhost: Add VIRTIO_F_IN_ORDER support

From: Jonah Palmer
Subject: Re: [RFC 0/8] virtio,vhost: Add VIRTIO_F_IN_ORDER support
Date: Tue, 26 Mar 2024 15:01:57 -0400
User-agent: Mozilla Thunderbird 


On 3/26/24 2:34 PM, Eugenio Perez Martin wrote:

On Tue, Mar 26, 2024 at 5:49 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:




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
     }
}
-----------



That's right.


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.



Totally correct.


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?


Maybe I explained something wrong, but virtqueue_fill should not call
virtqueue_push. It's up to the caller (virtio-net, virtio-blk, etc) to
call one or another. Can you elaborate?
Oh I see, my apologies! I misunderstood and thought you were suggesting to call virtqueue_push inside of virtqueue_fill. 


-----------

Lastly, when execution reaches virtqueue_flush, what would define an
element as unused? Perhaps...

if (vq->used_elems[i] == NULL)



used_elems is not an array of pointers but an array of
VirtQueueElement so we cannot do this way.


or

if (vq->used_elems[i].in_num + vq->used_elems[i].out_num > 0)



Right, I propose to reset both in_num = out_num = 0.

Thanks!



Gotcha. I'll take this and work on getting a v2 RFC out.

Thank you for the re-clarifying things again Eugenio!


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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