Re: [Qemu-devel] [PATCH 1/3] virtio: Introduce VirtIODevice.broken

[Fam Zheng]

On Sun, 04/27 11:59, Michael S. Tsirkin wrote:
> On Sun, Apr 27, 2014 at 09:34:06AM +0100, Peter Maydell wrote:
> > On 27 April 2014 09:29, Michael S. Tsirkin <address@hidden> wrote:
> > > On Tue, Apr 22, 2014 at 04:55:15PM +0800, Fam Zheng wrote:
> > >> If guest driver behaves abnormally, emulation code could mark the device
> > >> as "broken".
> > >>
> > >> Once "broken" is set, device emulation will typically wait for a reset
> > >> command and ignore any other operations, but it could also return error
> > >> responds. In other words, whether and how does guest know about this
> > >> error status is device specific.
> > >>
> > >> Signed-off-by: Fam Zheng <address@hidden>
> > >
> > > I'm assuming the idea is to make debugging guest drivers easier
> > > for people not familiar with qemu?
> > 
> > As a general rule, guests shouldn't be able to cause QEMU to
> > just randomly exit. We have a bunch of code in tree which does
> > handle guest errors this way, of course, but cleanups to fix it
> > are worth having.
> 
> OK so by using a wrong address an MMIO handler can e.g. start
> MMIO on the device itself instead of doing DMA,
> this will cause an infinite loop.
> Any idea how to fix this?
> 
> > The benefits include that one duff device
> > driver doesn't take out your whole VM, that you have a chance
> > for a clean shutdown, and reboot might restore the operation of
> > the offending device.
> > 
> > There was a thread about this a little while back.
> > 
> > thanks
> > -- PMM
> 
> I agree, but I'd like the change to be done in a way that does not make
> debugging harder.
> 

Michael, Why is debugging harder with this patch, please?

Fam