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
