From 19159a7f012b8f886c0e631003e4834145b62ac9 Mon Sep 17 00:00:00 2001 From: Thomas Huth Date: Wed, 16 Aug 2023 23:07:43 +0200 Subject: [PATCH] chardev/char-pty: Avoid losing bytes when the other side just (re-)connected MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Signed-off-by: Thomas Huth Reviewed-by: Daniel P. Berrangé Message-Id: <20230816210743.1319018-1-thuth@redhat.com> (cherry picked from commit 4f7689f0817a717d18cc8aca298990760f27a89b) Signed-off-by: Michael Tokarev --- chardev/char-pty.c | 22 +++++++++++++++++++--- 1 file changed, 19 insertions(+), 3 deletions(-) diff --git a/chardev/char-pty.c b/chardev/char-pty.c index 4e5deac18a..cc2f7617fe 100644 --- a/chardev/char-pty.c +++ b/chardev/char-pty.c @@ -106,11 +106,27 @@ static void pty_chr_update_read_handler(Chardev *chr) static int char_pty_chr_write(Chardev *chr, const uint8_t *buf, int len) { PtyChardev *s = PTY_CHARDEV(chr); + GPollFD pfd; + int rc; - if (!s->connected) { - return len; + if (s->connected) { + return io_channel_send(s->ioc, buf, len); } - return io_channel_send(s->ioc, buf, len); + + /* + * The other side might already be re-connected, but the timer might + * not have fired yet. So let's check here whether we can write again: + */ + pfd.fd = QIO_CHANNEL_FILE(s->ioc)->fd; + pfd.events = G_IO_OUT; + pfd.revents = 0; + rc = RETRY_ON_EINTR(g_poll(&pfd, 1, 0)); + g_assert(rc >= 0); + if (!(pfd.revents & G_IO_HUP) && (pfd.revents & G_IO_OUT)) { + io_channel_send(s->ioc, buf, len); + } + + return len; } static GSource *pty_chr_add_watch(Chardev *chr, GIOCondition cond)