From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 52C26C2FC29 for ; Thu, 17 Aug 2023 13:47:53 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qWdLj-00021h-0s; Thu, 17 Aug 2023 09:47:43 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qWdLX-0001uP-Mq for qemu-devel@nongnu.org; Thu, 17 Aug 2023 09:47:35 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qWdLT-0003O8-66 for qemu-devel@nongnu.org; Thu, 17 Aug 2023 09:47:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1692280045; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=UCXbO8fuv67kKHW2N4r2VmGJFrf4wlMWzcyUKp7sKOE=; b=WRL7xBoHT4QG/iJb1DLKOba15xWqsiLj442QqwV9iBKk1cGW8DKzRUiDJUbZxnGyQI6uMH EG5hkaFInC8HwlfvXmOzjBwDOq5V1WMM+dS61tRBkMjnPGG7TQqhOzPs5M84QO1CjRn7ay Ff0usXq1tmEqdU84ShTcdjx/DWGGgpc= Received: from mail-ej1-f69.google.com (mail-ej1-f69.google.com [209.85.218.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-345-6X88qH-0NOq1IMCW5C-UsA-1; Thu, 17 Aug 2023 09:47:24 -0400 X-MC-Unique: 6X88qH-0NOq1IMCW5C-UsA-1 Received: by mail-ej1-f69.google.com with SMTP id a640c23a62f3a-99beea69484so441554866b.0 for ; Thu, 17 Aug 2023 06:47:21 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1692280041; x=1692884841; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=UCXbO8fuv67kKHW2N4r2VmGJFrf4wlMWzcyUKp7sKOE=; b=Gh3J8TMnaeBtVh6lkuvaBK4S7oLldYJTwNB94LUXmEbcX4j0zFNvP8pxOcWmHwDxCB Ue5dBxOSCVuSSalaIrueJCm6QyoO3rkCu6380B3kPjVU1E8UtCRT+mGTEm+ijGc/bYJU d2IMwDK+V5kk6GJ9+nFz6AKx3GsR2uBd/qbgzW46VgikT3c85T/SSHBcLXyTgo8Qit+K 0IbSvhCKWKHeKffml3oh3VHGuWqeFxFH0liylQr8njJheXiuerq1SZbqohdxukgD8Z3W wnivMHY7P6ZZxPP8qJBs4S7ZJrNymN+h/VhRQPYxfgQpSNXvco5RgFr6wc4c4YxCFcoY YLfw== X-Gm-Message-State: AOJu0YzOpPOrD47J5V80/amerUBcik48m8HYM4JL2abjAhjT67O9mL3d brlFSX9Mf7utgi7zIJWtLdjgKmbBfoJlFiEzo7mTCi892sy7cnxiVHtn+0TuyLwceVpnFJcCWgq iC7GfCzyrabY8ao6nOCkvOYAAtypnUgI= X-Received: by 2002:a17:907:1df2:b0:99d:fc31:242f with SMTP id og50-20020a1709071df200b0099dfc31242fmr2840394ejc.66.1692280040978; Thu, 17 Aug 2023 06:47:20 -0700 (PDT) X-Google-Smtp-Source: AGHT+IFwIl8tPHcoAthrOXNDbAB2tXASOW+3dpZUzKyO036H5WprrF52HMzDkCg0Q04otydvzAMM1PBZs32lDaKB+Nk= X-Received: by 2002:a17:907:1df2:b0:99d:fc31:242f with SMTP id og50-20020a1709071df200b0099dfc31242fmr2840373ejc.66.1692280040648; Thu, 17 Aug 2023 06:47:20 -0700 (PDT) MIME-Version: 1.0 References: <20230816210743.1319018-1-thuth@redhat.com> <2bb2f8ac-43b4-9505-c163-d29964bf6a30@redhat.com> In-Reply-To: From: =?UTF-8?B?TWFyYy1BbmRyw6kgTHVyZWF1?= Date: Thu, 17 Aug 2023 17:47:09 +0400 Message-ID: Subject: Re: [PATCH] chardev/char-pty: Avoid losing bytes when the other side just (re-)connected To: =?UTF-8?Q?Daniel_P=2E_Berrang=C3=A9?= Cc: Thomas Huth , qemu-devel@nongnu.org, Paolo Bonzini , Marc Hartmayer Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=170.10.129.124; envelope-from=mlureau@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.021, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Hi On Thu, Aug 17, 2023 at 5:06=E2=80=AFPM Daniel P. Berrang=C3=A9 wrote: > > On Thu, Aug 17, 2023 at 02:00:26PM +0200, Thomas Huth wrote: > > On 17/08/2023 12.32, Daniel P. Berrang=C3=A9 wrote: > > > 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 especial= ly > > > > annoying on s390x that only has a text console by default and no gr= aphical > > > > 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 th= e > > > > libvirt side) is that QEMU only checks with a 1 second timer whethe= r > > > > 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 ch= ecked > > > > for a re-connection during write, but this has been removed in comm= it > > > > f8278c7d74 ("char-pty: remove the check for connection on write") t= o 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 h= as not > > > > been marked as "connected" yet. The file descriptor is marked as no= n-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 > > > > --- > > > > 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..fad12dfef3 100644 > > > > --- a/chardev/char-pty.c > > > > +++ b/chardev/char-pty.c > > > > @@ -106,11 +106,27 @@ static void pty_chr_update_read_handler(Chard= ev *chr) > > > > static int char_pty_chr_write(Chardev *chr, const uint8_t *buf, i= nt len) > > > > { > > > > PtyChardev *s =3D 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 writ= e again: > > > > + */ > > > > + pfd.fd =3D QIO_CHANNEL_FILE(s->ioc)->fd; > > > > + pfd.events =3D G_IO_OUT; > > > > + pfd.revents =3D 0; > > > > + rc =3D RETRY_ON_EINTR(g_poll(&pfd, 1, 0)); > > > > + g_assert(rc >=3D 0); > > > > + if (!(pfd.revents & G_IO_HUP) && (pfd.revents & G_IO_OUT)) { > > > > > > Should (can?) we call > > > > > > pty_chr_state(chr, 1); > > > > > > here ? > > > > As far as I understood commit f8278c7d74c6 and f7ea2038bea04628, this i= s not > > possible anymore since the lock has been removed. > > > > > > + io_channel_send(s->ioc, buf, len); > > > > > > As it feels a little dirty to be sending data before setting the > > > 'connected =3D=3D 1' and thus issuing the 'CHR_EVENT_OPENED' event > > > > I didn't find a really better solution so far. We could maybe introduce= a > > buffer in the char-pty code and store the last second of guest output, = but > > IMHO that's way more complex and thus somewhat ugly, too? > > The orignal commit f8278c7d74c6 said > > [quote] > char-pty: remove the check for connection on write > > This doesn't help much compared to the 1 second poll PTY > timer. I can't think of a use case where this would help. > [/quote] > > We've now identified a use case where it is actually important. > > IOW, there's a justification to revert both f7ea2038bea04628 and > f8278c7d74c6, re-adding the locking and write update logic. Indeed. But isn't it possible to watch for IO_OUT and get rid of the timer? Other thing I question is whether the serial shouldn't return BUSY if the chardev is disconnected..