Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET"

[Cornelia Huck]

On Tue, 21 Apr 2015 16:38:31 +0800
Fam Zheng <address@hidden> wrote:

> On Tue, 04/21 10:04, Cornelia Huck wrote:
> > On Tue, 21 Apr 2015 15:44:02 +0800
> > Fam Zheng <address@hidden> wrote:
> > 
> > > On Mon, 04/20 17:13, Cornelia Huck wrote:
> > > > On Fri, 17 Apr 2015 15:59:15 +0800
> > > > Fam Zheng <address@hidden> wrote:
> > > > 
> > > > > Currently, virtio code chooses to kill QEMU if the guest passes any 
> > > > > invalid
> > > > > data with vring. That has drawbacks such as losing unsaved data (e.g. 
> > > > > when
> > > > > guest user is writing a very long email), or possible denial of 
> > > > > service in
> > > > > a nested vm use case where virtio device is passed through.
> > > > > 
> > > > > virtio-1 has introduced a new status bit "NEEDS RESET" which could be 
> > > > > used to
> > > > > improve this by communicating the error state between virtio devices 
> > > > > and
> > > > > drivers. The device notifies guest upon setting the bit, then the 
> > > > > guest driver
> > > > > should detect this bit and report to userspace, or recover the device 
> > > > > by
> > > > > resetting it.
> > > > > 
> > > > > This series makes necessary changes in virtio core code, based on 
> > > > > which
> > > > > virtio-blk is converted. Other devices now keep the existing behavior 
> > > > > by
> > > > > passing in "error_abort". They will be converted in following series. 
> > > > > The Linux
> > > > > driver part will also be worked on.
> > > > > 
> > > > > One concern with this behavior change is that it's now harder to 
> > > > > notice the
> > > > > actual driver bug that caused the error, as the guest continues to 
> > > > > run.  To
> > > > > address that, we could probably add a new error action option to 
> > > > > virtio
> > > > > devices,  similar to the "read/write werror" in block layer, so the 
> > > > > vm could be
> > > > > paused and the management will get an event in QMP like pvpanic.  
> > > > > This work can
> > > > > be done on top.
> > > > 
> > > > In principle, this looks nice; I'm not sure however how this affects
> > > > non-virtio-1 devices.
> > > > 
> > > > If a device is operating in virtio-1 mode, everything is clearly
> > > > specified: The guest is notified and if it is aware of the NEEDS_RESET
> > > > bit, it can react accordingly.
> > > > 
> > > > But what about legacy devices? Even if they are notified, they don't
> > > > know to check for NEEDS_RESET - and I'm not sure if the undefined
> > > > behaviour after NEEDS_RESET might lead to bigger trouble than killing
> > > > off the guest.
> > > > 
> > > 
> > > The device should become unresponsive to VQ output until guest issues a 
> > > reset
> > > through bus commands.  Do you have an example of "big trouble" in mind?
> > 
> > I'm not sure what's supposed to happen if NEEDS_RESET is set but not
> > everything is fenced off. The guest may see that queues have become
> > unresponsive, but if we don't stop ioeventfds and fence off
> > notifications, it may easily get into an undefined state internally.
> 
> Yeah, disabling ioeventfds and notifications is a good idea.
> 
> > And if it is connected to other guests via networking, having it limp
> > on may be worse than just killing it off. (Which parts of the data have
> > been cleanly written to disk and which haven't?
> 
> Well, we don't know that even without this series, do we?

We know it hasn't, as the guest is dead :)

> 
> > How is it going to get
> > out of that pickle if it has no good idea of what is wrong?
> 
> If it's virtio-1 compatible, it can reset the device or mark the device
> ususable, either way guest gets a chance to save the work.

My problem is not with virtio-1 devices; although data certainly can't
be written if the device has become unusable.

> 
> If it's not, it's merely an unresponsive device, and guest user can
> reboot/shutdown.

But how does any management software know? If I'm logged into a system
and I notice that saving my data doesn't complete, I can trigger an
action (although reboot/shutdown may not work anymore if too many
threads are waiting on writeback), but how can an automation system
know? It is probably more useful for those setups to have a hard stop
if recovery is not possible - and for legacy systems, that means
killing the guest afaics.

> 
> > 
> > If I have to debug a non-working guest, I prefer a crashed one with a
> > clean state over one that has continued running after the error
> > occurred.
> 
> For debugging purpose, crashing is definitely fine (even better :), but only
> because we won't have critical applications in guest. 

I would argue even for critical applications. They should have a second
guest as backup :)

> It makes sense to user to
> avoid the overkiller "exit(1)"'s in QEMU. They don't even generate a core 
> file.

Let's keep dying, but use abort? Would that help?

> And even if they do, it would be much more painful to recover an unsaved
> libreoffice document from a memory core.

See my reply above.

My concern is mainly about legacy setups that aren't used interactively.