On 3/22/24 7:18 AM, Eugenio Perez Martin wrote:
On Thu, Mar 21, 2024 at 4:57 PM Jonah Palmer <jonah.pal...@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?
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
