Re: [Qemu-devel] [PATCH 1/2] virtio-9p: print error message and exit instead of BUG_ON()

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

On Thu, Sep 08, 2016 at 06:26:52PM +0200, Greg Kurz wrote:
> On Thu, 8 Sep 2016 18:19:27 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Thu, Sep 08, 2016 at 05:04:47PM +0200, Cornelia Huck wrote:
> > > On Thu, 8 Sep 2016 18:00:28 +0300
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > >   
> > > > On Thu, Sep 08, 2016 at 11:12:16AM +0200, Greg Kurz wrote:  
> > > > > On Thu, 8 Sep 2016 10:59:26 +0200
> > > > > Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
> > > > >   
> > > > > > On Wed, 07 Sep 2016 19:19:24 +0200
> > > > > > Greg Kurz <groug@kaod.org> wrote:
> > > > > >   
> > > > > > > Calling assert() really makes sense when hitting a genuine bug, which calls
> > > > > > > for a fix in QEMU. However, when something goes wrong because the guest
> > > > > > > sends a malformed message, it is better to write down a more meaningul
> > > > > > > error message and exit.
> > > > > > > 
> > > > > > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > > > > > ---
> > > > > > >  hw/9pfs/virtio-9p-device.c |   20 ++++++++++++++++++--
> > > > > > >  1 file changed, 18 insertions(+), 2 deletions(-)    
> > > > > > 
> > > > > > While this is an improvement over the current state, I don't think the
> > > > > > guest should be able to kill qemu just by doing something stupid.
> > > > > >   
> > > > > 
> > > > > Hi Connie,
> > > > > 
> > > > > I'm glad you're pointing this out... this was also my impression, but
> > > > > since there are a bunch of sanity checks in the virtio code that cause
> > > > > QEMU to exit (even recently added like 1e7aed70144b), I did not dare
> > > > > stand up :)  
> > > > 
> > > > It's true that it's broken in many places but we should just
> > > > fix them all.
> > > > 
> > > > 
> > > > A separate question is how to log such hardware/guest bugs generally.
> > > > People already complained about disk filling up because of us printing
> > > > errors on each such bug.  Maybe print each message only N times, and
> > > > then set a flag to skip the log until management tells us to restart
> > > > logging again.  
> > > 
> > > I'd expect to get the message just once per device if we set the device
> > > to broken (unless the guess continuously resets it again...)  
> > 
> > Which it can do, so we should limit that anyway.
> > 
> > > Do we have
> > > a generic print/log ratelimit infrastructure in qemu?  
> > 
> > There are actually two kinds of errors
> > host side ones and ones triggered by guests.
> > 
> > We should distinguish between them API-wise, then
> > we will be able to limit the logging of those
> > that guest can trigger.
> > 
> 
> FWIW it makes sense to use error_report() if QEMU exits.

Not necessarily e.g. hotplug errors trigger error_report too.
Generally it should be for host misconfiguration or similar
management errors.

> If it continues
> execution, this means we're expecting the guest or the host to do something
> to fix the error condition. This requires QEMU to emit an event of some
> sort, but not necessarily to log an error message in a file. I guess this
> depends if QEMU is run by some tooling, or by a human.

I'm not sure we need an event if tools are not expected to
do anything with it. If we limit # of times error
is printed, tools will need to reset this counter,
so we will need an event on overflow.


-- 
MST