[dpdk-dev] virtio with 2MB hugepages - bringing back single file segments

[Maxime Coquelin]

On 03/02/2018 10:03 AM, Stojaczyk, DariuszX wrote:
> Hi Maxime,
> 
>>> Hi,
>>>
>>> I'm trying to make a vhost-user initiator built upon DPDK work with
>>> 2MB hugepages. In the initiator we have to share all memory with the host
>> process, so it can perform DMA. DPDK currently enforces having one descriptor
>> per hugepage and there's an artificial limit of shared descriptors in DPDK
>> vhost-user implementation (currently 8). Because of that, all DPDK vhost-user
>> initiators are practically limited to 1GB hugepages at the moment. We can
>> always increase the artificial descriptor limit, but then we're limited by
>> sendmsg() itself, which on Linux accepts no more than 253 descriptors.
>> However, could we increase the vhost-user implementation limit to - say - 128,
>> and bring back "single file segments" [1]?
>>
>> If you do something like this, you'll have first to update the vhost-user spec,
>> which should I think include a new protocol feature bit.
> 
> Do I? I can't see any memory region limitation in the vhost-user spec [1]. If a 128-region initiator tries to connect to an 8-region rte_vhost, the recvmsg() would simply fail - rte_vhost rejects `truncated` messages.

Here it is:

* Memory regions description
    ---------------------------------------------------
    | num regions | padding | region0 | ... | region7 |
    ---------------------------------------------------

    Num regions: a 32-bit number of regions
    Padding: 32-bit


> 
>>
>> Also, you will have to consider improving the translation functions with a better
>> search algorithm, else you'll have very poor performance.
> 
> That's right, I got that sorted out already.
> 
> Regards,
> D.
> 
> [1] https://github.com/qemu/qemu/blob/master/docs/interop/vhost-user.txt
>