[Qemu-devel] char: Logging serial pty output when disconnected

[Ed Swierk]

Interactive access to a guest serial console can be enabled by hooking
the serial device to a pty backend, e.g. -device
isa-serial,chardev=cs0 -chardev pty,id=cs0. With libvirt this can be
configured via <console type='pty'><target type='serial'
port='0'/></console>.

Output from the same serial device can also be logged to a file by
adding logfile=/somefile to the -chardev option (<log file=/somefile/>
in libvirt).

Unfortunately output gets logged only when a client like virsh console
is connected to the pty; otherwise qemu drops it on the floor. This
makes chardev logging much less useful than it could be for debugging
guest problems after the fact.

Currently qemu_chr_fe_write() calls qemu_chr_fe_write_log() only for
data consumed by the backend chr_write function. With the pty backend,
pty_chr_write() returns 0 indicating that the data was not consumed
when the pty is disconnected. Simply changing it to return len instead
of 0 tricks the caller into logging the data even when the pty is
disconnected. I don't know what problems this might cause, but one
data point is that tcp_chr_write() already happens to work this way.

Alternatively, qemu_chr_fe_write() could be modified to log everything
passed to it, regardless of how much data chr_write claims to have
consumed. The trouble is that the serial device retries writing
unconsumed data, so when the pty is disconnected you'd see every
character duplicated 4 times in the log file.

Any opinions on either approach, or other suggestions? If there are no
objections to the first one, I'll prepare a patch.

--Ed