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Fri, 31 Jul 2020 15:18:40 +0000 Received: from abhmp0002.oracle.com (abhmp0002.oracle.com [141.146.116.8]) by userv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 06VFIcwL019069; Fri, 31 Jul 2020 15:18:38 GMT Received: from [10.39.235.87] (/10.39.235.87) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 31 Jul 2020 08:18:38 -0700 Subject: Re: [PATCH V1 12/32] vl: pause option To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <1596122076-341293-1-git-send-email-steven.sistare@oracle.com> <1596122076-341293-13-git-send-email-steven.sistare@oracle.com> <1dea1698-f8be-519d-e00c-d163b08dca65@redhat.com> <36036b5f-4e63-4287-a8e6-499732f54689@oracle.com> <20200731100742.GA3641941@redhat.com> From: Steven Sistare Organization: Oracle Corporation Message-ID: <0b41c5b4-8d06-be87-d814-f7f39b43f2d7@oracle.com> Date: Fri, 31 Jul 2020 11:18:35 -0400 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <20200731100742.GA3641941@redhat.com> Content-Type: text/plain; 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Tsirkin" , Juan Quintela , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , =?UTF-8?Q?Alex_Benn=c3=a9e?= , Alex Williamson , Stefan Hajnoczi , =?UTF-8?Q?Marc-Andr=c3=a9_Lureau?= , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , Markus Armbruster Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 7/31/2020 6:07 AM, Daniel P. Berrangé wrote: > 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 That is a nice solution for the launchers we can modify. We could use your idea in place of the sleep in main, kill(getpid(), SIGSTOP); Not quite as good as being able to debug the elf constructors, but still helpful. - Steve