On Tue, 21 Apr 2015 16:38:31 +0800 Fam Zheng <f...@redhat.com> wrote:
> On Tue, 04/21 10:04, Cornelia Huck wrote: > > On Tue, 21 Apr 2015 15:44:02 +0800 > > Fam Zheng <f...@redhat.com> wrote: > > > > > On Mon, 04/20 17:13, Cornelia Huck wrote: > > > > On Fri, 17 Apr 2015 15:59:15 +0800 > > > > Fam Zheng <f...@redhat.com> 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.