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

[Daniel P. Berrange]

On Thu, Jan 26, 2017 at 05:07:16PM -0800, Ed Swierk wrote:
> 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.

If the pty backend intends to just drop data into a blackhole when
no client is connected, then its chr_write() impl should return
the length of the data discarded, not zero.

Regards,
Daniel
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