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

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

On Fri, Apr 17, 2015 at 03:59:15PM +0800, Fam Zheng 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.

Unfortunately, virtio 1 spec does not have a conformance statement
that requires driver to recover. We merely have a non-normative looking
text:
	Note: For example, the driver can’t assume requests in flight
	will be completed if DEVICE_NEEDS_RESET is set, nor can it assume that
	they have not been completed. A good implementation will try to recover
	by issuing a reset.

Implementing this reset for all devices in a race-free manner might also
be far from trivial.  I think we'd need a feature bit for this.
OTOH as long as we make this a new feature, would an ability to
reset a single VQ be a better match for what you are trying to
achieve?

> 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.

At the architectural level, that's only one concern. Others would be
- workloads such as openstack handle guest crash better than
  a guest that's e.g. slow because of a memory leak
- it's easier for guests to probe host for security issues
  if guest isn't killed
- guest can flood host log with guest-triggered errors


At the implementation level, there's one big issue you seem to have
missed: DMA to invalid memory addresses causes a crash in memory core.
I'm not sure whether it makes sense to recover from virtio core bugs
when we can't recover from device bugs.


> 
> 
> Fam Zheng (18):
>   virtio: Return error from virtqueue_map_sg
>   virtio: Return error from virtqueue_num_heads
>   virtio: Return error from virtqueue_get_head
>   virtio: Return error from virtqueue_next_desc
>   virtio: Return error from virtqueue_get_avail_bytes
>   virtio: Return error from virtqueue_pop
>   virtio: Return error from virtqueue_avail_bytes
>   virtio: Return error from virtio_add_queue
>   virtio: Return error from virtio_del_queue
>   virtio: Add macro for VIRTIO_CONFIG_S_NEEDS_RESET
>   virtio: Add "needs_reset" flag to virtio device
>   virtio: Return -EINVAL if the vdev needs reset in virtqueue_pop
>   virtio-blk: Graceful error handling of virtqueue_pop
>   qtest: Add "QTEST_FILTER" to filter test cases
>   qtest: virtio-blk: Extract "setup" for future reuse
>   libqos: Add qvirtio_needs_reset
>   qtest: Add test case for "needs reset" of virtio-blk
>   qtest: virtio-blk: Suppress virtio error messages in "make check"
> 
>  hw/9pfs/virtio-9p-device.c                     |   2 +-
>  hw/9pfs/virtio-9p.c                            |   2 +-
>  hw/block/dataplane/virtio-blk.c                |   9 +-
>  hw/block/virtio-blk.c                          |  62 +++++--
>  hw/char/virtio-serial-bus.c                    |  30 ++--
>  hw/net/virtio-net.c                            |  36 +++--
>  hw/scsi/virtio-scsi.c                          |   8 +-
>  hw/virtio/virtio-balloon.c                     |  13 +-
>  hw/virtio/virtio-rng.c                         |   6 +-
>  hw/virtio/virtio.c                             | 214 ++++++++++++++++++-------
>  include/hw/virtio/virtio-blk.h                 |   3 +-
>  include/hw/virtio/virtio.h                     |  17 +-
>  include/standard-headers/linux/virtio_config.h |   2 +
>  tests/Makefile                                 |   6 +-
>  tests/libqos/virtio.c                          |   5 +
>  tests/libqos/virtio.h                          |   2 +
>  tests/virtio-blk-test.c                        | 196 ++++++++++++++++++++--
>  17 files changed, 482 insertions(+), 131 deletions(-)
> 
> -- 
> 1.9.3
>