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Subject: Re: [PATCH v8 00/10] Introduce VDUSE - vDPA Device in Userspace
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From:   Jason Wang <jasowang@redhat.com>
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Date:   Tue, 29 Jun 2021 12:10:51 +0800
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在 2021/6/28 下午1:54, Liu, Xiaodong 写道:
>> Several issues:
>>
>> - VDUSE needs to limit the total size of the bounce buffers (64M if I was not
>> wrong). Does it work for SPDK?
> Yes, Jason. It is enough and works for SPDK.
> Since it's a kind of bounce buffer mainly for in-flight IO, so limited size like
> 64MB is enough.


Ok.


>
>> - VDUSE can use hugepages but I'm not sure we can mandate hugepages (or we
>> need introduce new flags for supporting this)
> Same with your worry, I'm afraid too that it is a hard for a kernel module
> to directly preallocate hugepage internal.
> What I tried is that:
> 1. A simple agent daemon (represents for one device)  `preallocates` and maps
>      dozens of 2MB hugepages (like 64MB) for one device.
> 2. The daemon passes its mapping addr&len and hugepage fd to kernel
>      module through created IOCTL.
> 3. Kernel module remaps the hugepages inside kernel.


Such model should work, but the main "issue" is that it introduce  
overheads in the case of vhost-vDPA.

Note that in the case of vhost-vDPA, we don't use bounce buffer, the  
userspace pages were shared directly.

And since DMA is not done per page, it prevents us from using tricks  
like vm_insert_page() in those cases.


> 4. Vhost user target gets and maps hugepage fd from kernel module
>      in vhost-user msg through Unix Domain Socket cmsg.
> Then kernel module and target map on the same hugepage based
> bounce buffer for in-flight IO.
>
> If there is one option in VDUSE to map userspace preallocated memory, then
> VDUSE should be able to mandate it even it is hugepage based.
>

As above, this requires some kind of re-design since VDUSE depends on  
the model of mmap(MAP_SHARED) instead of umem registering.

Thanks