On 10/25/22 17:44, Bruce Richardson wrote:
On Tue, Oct 25, 2022 at 05:33:31PM +0200, Maxime Coquelin wrote:
On 10/25/22 11:15, Hu, Jiayu wrote:
I think that for Vhost PMD, the Virtio completions should either be
performed by DMA engine or by a dedicated thread.
We cannot depend on DMA engine to do completion, as there is no
ordering guarantee on the HW. For example, given the DMA engine
issues two updates on the used ring's index, it is possible that the second
write completes before the first one.
I'm not sure for Intel hardware, but other vendors may offer ordering
guarantees, it should be exposed as a capability of the DMA device. If
the DMA device offers this capability, it could be used for Vhost.
While I haven't been following this discussion, this particular comment
caught my eye.
For jobs submitted via a single dmadev device, the "FENCE" flag is provided
as part of the dmadev API[1]. Obviously, if the writes come from different
dmadevs, then things are rather more complicated.
Thanks for the clarification Bruce.
In the Vhost PMD case, there is a 1:1 mapping between the virtqueue and
the DMA channel, so we should be fine.
Regards,
Maxime
/Bruce
[1]
https://doc.dpdk.org/api/rte__dmadev_8h.html#a3375e7b956b305505073c4ff035afe2f
