Re: [PATCH] vhost: flush used->idx update before reading avail->flags

[Linhaifeng <haifeng.lin@huawei.com>]

On 2015/6/10 16:30, Luke Gorrie wrote:
> On 9 June 2015 at 10:46, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>> By the way, similarly, host side must re-check avail idx after writing
>> used flags. I don't see where snabbswitch does it - is that a bug
>> in snabbswitch?
> 
> 
> Good question.
> 
> Snabb Switch does not use interrupts from the guest. We always set
> VRING_F_NO_NOTIFY to tell the guest that it need not interrupt us. Then we
> run in poll mode and in practice check the avail ring for new descriptors
> every 20us or so.

Yes, host not need guest to notify when in poll mode but host also need to
notify guest who use virtio_net when vring is full or emtpy.If host loss this
notification guest would stop working.

> 
> So the argument for not needing this check in both Snabb Switch and DPDK is
> that we are running poll mode and don't notice whether interrupts are being
> sent or not.
> 
> Is that a solid argument or do I misunderstand what the race condition is?
> 
> Cheers,
> -Luke
>