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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 09, 2015 at 03:04:02PM +0800, Linhaifeng wrote:
> 
> 
> On 2015/4/24 15:27, Luke Gorrie wrote:
> > On 24 April 2015 at 03:01, Linhaifeng <haifeng.lin@huawei.com> wrote:
> > 
> >> If not add memory fence what would happen? Packets loss or interrupt
> >> loss?How to test it ?
> >>
> > 
> > You should be able to test it like this:
> > 
> > 1. Boot two Linux kernel (e.g. 3.13) guests.
> > 2. Connect them via vhost switch.
> > 3. Run continuous traffic between them (e.g. iperf).
> > 
> > I would expect that within a reasonable timeframe (< 1 hour) one of the
> > guests' network interfaces will hang indefinitely due to a missed interrupt.
> > 
> > You won't be able to reproduce this using DPDK guests because they are not
> > using the same interrupt suppression method.
> > 
> > This is a serious real-world problem. I wouldn't deploy the vhost
> > implementation without this fix.
> > 
> > Cheers,
> > -Luke
> > 
> 
> I think this patch can't resole this problem. On the other hand we still would miss interrupt.
> 
> After add rte_mb() function the we want the case is :
> 1.write used->idx. ring is full or empty.
> 2.virtio_net open interrupt.
> 3.read avail->flags.
> 
> but this case(miss interrupt) would happen too:
> 1.write used->idx. ring is full or empty.
> 2.read avail->flags.
> 3.virtio_net open interrupt.
> 

That's why a correct guest, after detecting an empty used ring, must always
re-check used idx at least once after writing avail->flags.

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?

-- 
MST