Re: [PATCH] chardev/char-pty: Avoid losing bytes when the other side just (re-)connected

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Aug 16, 2023 at 11:07:43PM +0200, Thomas Huth wrote:
> When starting a guest via libvirt with "virsh start --console ...",
> the first second of the console output is missing. This is especially
> annoying on s390x that only has a text console by default and no graphical
> output - if the bios fails to boot here, the information about what went
> wrong is completely lost.
> 
> One part of the problem (there is also some things to be done on the
> libvirt side) is that QEMU only checks with a 1 second timer whether
> the other side of the pty is already connected, so the first second of
> the console output is always lost.
> 
> This likely used to work better in the past, since the code once checked
> for a re-connection during write, but this has been removed in commit
> f8278c7d74 ("char-pty: remove the check for connection on write") to avoid
> some locking.
> 
> To ease the situation here at least a little bit, let's check with g_poll()
> whether we could send out the data anyway, even if the connection has not
> been marked as "connected" yet. The file descriptor is marked as non-blocking
> anyway since commit fac6688a18 ("Do not hang on full PTY"), so this should
> not cause any trouble if the other side is not ready for receiving yet.
> 
> With this patch applied, I can now successfully see the bios output of
> a s390x guest when running it with "virsh start --console" (with a patched
> version of virsh that fixes the remaining issues there, too).
> 
> Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  chardev/char-pty.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


With regards,
Daniel
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