Re: [PATCH V2] vl: pause option

[Alex Bennée]

Eric Blake <eblake@redhat.com> writes:

> On 11/2/20 9:50 AM, Steve Sistare wrote:
>> Provide the -pause command-line parameter and the QEMU_PAUSE environment
>> variable to pause QEMU during process startup using SIGSTOP and allow a
>> developer to attach a debugger, or observe the process using tools such as
>> strace.  Useful when the QEMU has been launched with some other entity, such
>> as libvirt.  QEMU_PAUSE is checked in a constructor at the highest priority,
>> and can be used to debug other constructors.  The -pause option is checked
>> later, during argument processing in main, but is useful if passing an
>> environment variable from a launcher to qemu is awkard.
>> 
>> Usage: qemu -pause, or QEMU_PAUSE=1
>> After attaching a debugger, send SIGCONT to the qemu process to continue.
>
> Changing behavior via a new environment variable is awkward.  What
> happens, for example, if libvirt inherits this variable set, but is not
> aware of its impact to alter how qemu starts up?  Can we get by with
> ONLY a command line option?
>
> Also, how does this option differ from what we already have with qemu
> --preconfig?

In the original discussion:

  Subject: [PATCH V1 12/32] vl: pause option
  Date: Thu, 30 Jul 2020 08:14:16 -0700
  Message-Id: <1596122076-341293-13-git-send-email-steven.sistare@oracle.com>

it seems the idea was to stop qemu as early as possible for debugging
when launched by some other launcher which wasn't modifiable except by
pass through configuration to QEMU's command line.

<snip>

> Isn't it always possible to provide a wrapper qemu process to be invoked
> by whatever third-party management app (like libvirt), where your
> wrapper then starts the real qemu under gdb to begin with, rather than
> having to hack qemu itself to have a special start-up mode?

I agree - this feels like a bit of an over complication as a debug
helper. How many times can a failure to launch by some binary blob not
be debugged by either a gdb follow-fork or a copying of the command line
and running gdb --args?

-- 
Alex Bennée