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

[Fam Zheng]

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?

Fam