linux-kernel - SysRQ via serial console doesn't work for VMWare guests

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Message-ID: <CA+QCeVQC-iqPqmvKK6D6YL=4do8yB3ez3n=L2Jj9_2qu5FVLbQ@mail.gmail.com>
Date:	Mon, 3 Feb 2014 17:33:39 +0200
From:	Sergey Meirovich <rathamahata@...il.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: SysRQ via serial console doesn't work for VMWare guests

Hi,

I have tried to utilize SysRQ via serial console in VMWare environment:
>From what I've seen RS-232 Break + <KEY> just doesn't work for 3.13.1
for VMWare guests:

[root@...-eccs-sbx-db2 ~]# cat /proc/tty/driver/serial
serinfo:1.0 driver revision:
0: uart:16550A port:000003F8 irq:4 tx:10034 rx:1 RTS|CTS|DTR|DSR|CD
1: uart:16550A port:000002F8 irq:3 tx:0 rx:0 CTS|DSR|CD
2: uart:unknown port:000003E8 irq:4
3: uart:unknown port:000002E8 irq:3
[root@...-eccs-sbx-db2 ~]#

Kernel console messages are visible just fine. But SysRQ over serial
is not working.

What I have tried to issue break is:
1) screen Control-a b (first VM serial port connected to second VM serial port))
2) minicom Ctrl-a f (first VM serial port connected to second VM serial port)
3) telnet - ctrl + ] "send brk" from virtual serial port concentrator.

All three above seem like really sending break, though any SysRQ
command neither after 5 seconds as stated in documentation nor
immediately is producing any result.

Have anyone succeed to use SysRQ over serial with WMWare guests?

I have already opened Red Hat case for that. Their responce was:

"...I think this only works on real physical serial ports.  From the kernel
docs:
    On the serial console (PC style standard serial ports only) -
           You send a BREAK, then within 5 seconds a command key.
           Sending BREAK twice is interpreted as a normal BREAK.
https://www.kernel.org/doc/Documentation/sysrq.txt

An RS-232 BREAK is a period of no electricity on the wire.  That's
rather difficult to do in a software-only virtual environment..."

I have found that explanation a bit strange.
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