Re: [Qemu-devel] [PATCH for-2.4] Revert "vhost-user: Send VHOST_RESET_OWNER on vhost stop"

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

On Tue, Aug 02, 2016 at 11:42:01AM +0200, Luke Gorrie wrote:
> Hi Michael & all,
> 
> On 4 August 2015 at 21:00, Luke Gorrie <luke@snabb.co> wrote:
> 
>     Hi Michael,
> 
>     Sorry I didn't see this mail sooner -
> 
>     On 30 July 2015 at 10:36, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
>         This reverts commit 294ce717e0f212ed0763307f3eab72b4a1bdf4d0.
> 
>         vhost stop happens e.g. when guest unloads the driver,
>         so closing the backend connection is not the right
>         thing to do here.
> 
>         VHOST_RESET_OWNER should happen on vhost_dev_cleanup - it's
>         the counterpart of VHOST_SET_OWNER.
> 
>         Cc: Luke Gorrie <luke@snabb.co>
>         Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>         ---
> 
>         I think we need this in 2.4 to avoid introducing regressions
>         in the protocol. We'll fix properly in 2.5.
>         Luke, can you comment please?
> 
> 
>     Interesting. Currently we don't have a test case in Snabb Switch CI for
>     guests reloading drivers. We need to add that and let you know how it goes.
> 
> 
> We have this test case now. Took a year to add, almost to the day, but here we
> are :-).
> 
> Specifically we have beefed up our CI for Snabb to run some tens of thousands
> of end-to-end tests with VMs nightly. This includes testing a bunch of
> different QEMU versions with a bunch of different Snabb (host) and DPDK (guest)
> versions. It also exercises the transition where the guest first initializes a
> Virtio-net device with the kernel driver and then later hands the device over
> to the DPDK driver.
> 
> The data we have now shows that this test case is only working reliably with
> QEMU 2.4.1. My hypothesis is that this QEMU version is sending the
> VHOST_RESET_OWNER when the kernel driver shuts down - so that vswitch knows not
> to process descriptors - while the other versions are skipping this (because
> the patch was not introduced yet, or because the patch was reverted).
> 
> Here is the full thread on Github, becoming more QEMU-oriented as you scroll
> down:
> https://github.com/snabbco/snabb/issues/976#issuecomment-236838883
> 
> So, question for QEMU upstream, is there currently a supported way for the host
> vswitch to detect when the vrings are invalid? How should we operate to be safe
> from serving garbage DMA requests during guest driver resets and reboots?
> 
> Cheers and sorry for the e-mail latency :-)
> -Luke
> 
> 


I think this is adequately answered here:
http://git.qemu.org/?p=qemu.git;a=blob;f=docs/specs/vhost-user.txt;hb=HEAD#l149

HTH

-- 
MST