[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEED
From: |
Cornelia Huck |
Subject: |
Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET" |
Date: |
Tue, 21 Apr 2015 11:08:47 +0200 |
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.
- [Qemu-devel] [PATCH 18/18] qtest: virtio-blk: Suppress virtio error messages in "make check", (continued)
- [Qemu-devel] [PATCH 08/18] virtio: Return error from virtio_add_queue, Fam Zheng, 2015/04/17
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Cornelia Huck, 2015/04/20
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Fam Zheng, 2015/04/21
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Cornelia Huck, 2015/04/21
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Fam Zheng, 2015/04/21
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET",
Cornelia Huck <=
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Fam Zheng, 2015/04/21
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Cornelia Huck, 2015/04/21
- Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Michael S. Tsirkin, 2015/04/21
Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Michael S. Tsirkin, 2015/04/20
Re: [Qemu-devel] [PATCH 00/18] virtio-blk: Support "VIRTIO_CONFIG_S_NEEDS_RESET", Fam Zheng, 2015/04/20