Re: [PATCH V1 12/32] vl: pause option

[Daniel P . Berrangé]

On Thu, Jul 30, 2020 at 02:11:19PM -0400, Steven Sistare wrote:
> On 7/30/2020 12:20 PM, Eric Blake wrote:
> > On 7/30/20 10:14 AM, Steve Sistare wrote:
> >> Provide the -pause command-line parameter and the QEMU_PAUSE environment
> >> variable to briefly pause QEMU in main and allow a developer to attach gdb.
> >> Useful when the developer does not invoke QEMU directly, such as when using
> >> libvirt.
> > 
> > How would you set this option with libvirt?
> 
> Add -pause in the qemu args in the xml.
>  
> > It feels like you are trying to reinvent something that is already 
> > well-documented:
> > 
> > https://www.berrange.com/posts/2011/10/12/debugging-early-startup-of-kvm-with-gdb-when-launched-by-libvirtd/
> 
> Too many steps to reach BINGO for my taste.  Easier is better.  Also, in our 
> shop we start qemu 
> in other ways, such as via services.


A "sleep" is a pretty crude & unreliable way to get into debugging
though. It is racy for a start, but also QEMU has a bunch of stuff
that runs via ELF constructors before main() even starts.

So I feel like the thing that starts QEMU is better placed to provide
a way in for debugging.

eg the service launcher can send SIGSTOP to the child process immediately
before the execve(qemu) call.

Now user can attach with the debugger, allow execution to continue,
and has ability to debug *everything* right from the ELF constructors
onwards into main() and all that follows.

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|