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

[Fam Zheng]

On Tue, 04/21 11:08, Cornelia Huck wrote:
> 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.
> 

How about pausing guest and generating an QMP event?

Fam