Re: [Qemu-devel] [Question] dump memory when host pci device is used by guest

[Richard W.M. Jones]

On Mon, Oct 10, 2011 at 10:10:21AM +0100, Daniel P. Berrange wrote:
> On Mon, Oct 10, 2011 at 10:08:26AM +0100, Daniel P. Berrange wrote:
> > On Mon, Oct 10, 2011 at 10:21:02AM +0800, Wen Congyang wrote:
> > > At 10/09/2011 06:23 PM, Richard W.M. Jones Write:
> > > > On Sun, Oct 09, 2011 at 10:49:57AM +0200, Jan Kiszka wrote:
> > > >> As explained in the other replies: It is way more future-proof to use 
> > > >> an
> > > >> interface for this which was designed for it (remote gdb) instead of
> > > >> artificially relaxing reasonable constraints of the migration mechanism
> > > >> plus having to follow that format with the post-processing tool.
> > > > 
> > > > Any interface that isn't "get this information off my production
> > > > server *now*" so that I can get the server restarted, and send it to
> > > > an expert to analyse -- is a poor interface, whether it was designed
> > > > like that or not.  Perhaps we don't have the right interface at all,
> > > > but remote gdb is not it.
> > > 
> > > What about the following idea?
> > > 
> > > Introduce a new monitor command named dump, and this command accepts a 
> > > filename.
> > > We can use almost all migration's code. We use this command to dump 
> > > guest's
> > > memory, so there is no need to check whether the guest has a unmigratable 
> > > device.
> > 
> > I think it would be a good idea of QEMU had a dedicated 'dump' command
> > for this purpose, even if it was just an alias for 'migrate' initially.
> > I have never really liked the fact that we abuse the 'migrate' command
> > to generate a core dump. The resulting data file from this is more
> > complex than it really needs to be, causing complexity for post-processing
> > it. The needs of migration, are not entirely aligned with the needs of
> > core dumping in the long term, so we should allow the possibility of
> > their impls diverging without impacting apps using them.
> > 
> > So adding a 'dump' command which wrote out data in a format that was
> > optimized for offline processing by tools like 'crash' (or the windows
> > equivalent) would be a good improvement, even if it just reuses the
> > migrate code for now.
> 
> The other reason why it would be good, is that we would then have a clearly
> defined standard "QEMU dump format", instead of "libvirt dump format for QEMU"

Other reasons why this is a good idea:

 - so you don't need 3 processes doing a dance in order to capture a
   core dump

 - so that qemu can initiate a core dump itself (from watchdog events)

 - so that two qemu processes can do a core dump at the same time
   without trying to use conflicting ports

Rich.

-- 
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