Re: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio block device

[Liu Yuan <namei.unix@gmail.com>]

> It looks like the patch wouldn't work for testing multiple devices.
>
> vhost_blk_open() does
> +       used_info_cachep = KMEM_CACHE(used_info, SLAB_HWCACHE_ALIGN |
> SLAB_PANIC);
>

This is weird. how do you open multiple device?I just opened the device 
with following command:

-drive file=/dev/sda6,if=virtio,cache=none,aio=native -drive 
file=~/data0.img,if=virtio,cache=none,aio=native -drive 
file=~/data1.img,if=virtio,cache=none,aio=native

And I didn't meet any problem.

this would tell qemu to open three devices, and pass three FDs to three 
instances of vhost_blk module.
So KMEM_CACHE() is okay in vhost_blk_open().

> When opening second device, we get panic since used_info_cachep is
> already created. Just to make progress I moved this call to
> vhost_blk_init().
>
> I don't see any host panics now. With single block device (dd),
> it seems to work fine. But when I start testing multiple block
> devices I quickly run into hangs in the guest. I see following
> messages in the guest from virtio_ring.c:
>
> virtio_blk virtio2: requests: id 0 is not a head !
> virtio_blk virtio1: requests: id 0 is not a head !
> virtio_blk virtio4: requests: id 1 is not a head !
> virtio_blk virtio3: requests: id 39 is not a head !
>
> Thanks,
> Badari
>
>

vq->data[] is initialized by guest virtio-blk driver and vhost_blk is 
unware of it. it looks like used ID passed
over by vhost_blk to guest virtio_blk is wrong, but, it should not 
happen. :|

And I can't reproduce this on my laptop. :(

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