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 C6C72CAC592 for ; Mon, 15 Sep 2025 18:42:58 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uyE8J-00014R-A8; Mon, 15 Sep 2025 14:40:59 -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 1uyE8E-000149-KC for qemu-devel@nongnu.org; Mon, 15 Sep 2025 14:40:54 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uyE87-0003bK-Pn for qemu-devel@nongnu.org; Mon, 15 Sep 2025 14:40:54 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1757961641; h=from:from:reply-to: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=yy3hWTKz1yeMWBg/dSxzWEOrMKxDIju37VQ+gDy7Ots=; b=ZhWlVivc8H672lh044/+fzaQ7XgLbHbxq+15TYMuFvKOtQON0Mj9T7T5oDubqWLlNktqlV jbEJzAgFAIJlCrqrKm1maDYWzhlM7IF6hp4M7NgJjaQkoeWe5RkqwCLcsUj4U1dT88/eAz addl5pamgI9lFpYT8cFh+tf93YINA0I= Received: from mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-638-ae1JozfLM7anKxfYt2LCFQ-1; Mon, 15 Sep 2025 14:40:40 -0400 X-MC-Unique: ae1JozfLM7anKxfYt2LCFQ-1 X-Mimecast-MFC-AGG-ID: ae1JozfLM7anKxfYt2LCFQ_1757961639 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4090219107DD; Mon, 15 Sep 2025 18:40:39 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.50]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6500919560A2; Mon, 15 Sep 2025 18:40:36 +0000 (UTC) Date: Mon, 15 Sep 2025 19:40:33 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: qemu-devel@nongnu.org, Juraj Marcin , Fabiano Rosas Subject: Re: [PATCH v2 2/3] io/tls: Make qio_channel_tls_bye() always synchronous Message-ID: References: <20250911212355.1943494-1-peterx@redhat.com> <20250911212355.1943494-3-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.14 (2025-02-20) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@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=-0.035, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, T_SPF_TEMPERROR=0.01 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Sep 12, 2025 at 11:36:51AM -0400, Peter Xu wrote: > On Fri, Sep 12, 2025 at 12:27:52PM +0100, Daniel P. Berrangé wrote: > > On Thu, Sep 11, 2025 at 05:23:54PM -0400, Peter Xu wrote: > > > No issue I hit, the change is only from code observation when I am looking > > > at a TLS premature termination issue. > > > > > > qio_channel_tls_bye() API needs to be synchronous. When it's not, the > > > previous impl will attach an asynchronous task retrying but only until when > > > the channel gets the relevant GIO event. It may be problematic, because the > > > caller of qio_channel_tls_bye() may have invoked channel close() before > > > that, leading to premature termination of the TLS session. > > > > > > Remove the asynchronous handling, instead retry it immediately. Currently, > > > the only two possible cases that may lead to async task is either INTERRUPT > > > or EAGAIN. It should be suffice to spin retry as of now, until a solid > > > proof showing that a more complicated retry logic is needed. > > > > > > With that, we can remove the whole async model for the bye task. > > > > > > When at it, making the function return bool, which looks like a common > > > pattern in QEMU when errp is used. > > > > > > Side note on the tracepoints: previously the tracepoint bye_complete() > > > isn't used. Start to use it in this patch. bye_pending() and bye_cancel() > > > can be dropped now. Adding bye_retry() instead. > > > > > > Signed-off-by: Peter Xu > > > --- > > > include/io/channel-tls.h | 5 ++- > > > io/channel-tls.c | 86 +++++----------------------------------- > > > io/trace-events | 3 +- > > > 3 files changed, 15 insertions(+), 79 deletions(-) > > > > > > diff --git a/io/channel-tls.c b/io/channel-tls.c > > > index 5a2c8188ce..8510a187a8 100644 > > > --- a/io/channel-tls.c > > > +++ b/io/channel-tls.c > > > @@ -253,84 +253,25 @@ void qio_channel_tls_handshake(QIOChannelTLS *ioc, > > > qio_channel_tls_handshake_task(ioc, task, context); > > > } > > > > > > -static gboolean qio_channel_tls_bye_io(QIOChannel *ioc, GIOCondition condition, > > > - gpointer user_data); > > > - > > > -static void qio_channel_tls_bye_task(QIOChannelTLS *ioc, QIOTask *task, > > > - GMainContext *context) > > > +bool qio_channel_tls_bye(QIOChannelTLS *ioc, Error **errp) > > > { > > > - GIOCondition condition; > > > - QIOChannelTLSData *data; > > > int status; > > > - Error *err = NULL; > > > > > > - status = qcrypto_tls_session_bye(ioc->session, &err); > > > + trace_qio_channel_tls_bye_start(ioc); > > > +retry: > > > + status = qcrypto_tls_session_bye(ioc->session, errp); > > > > > > if (status < 0) { > > > trace_qio_channel_tls_bye_fail(ioc); > > > > snip > > > > > + return false; > > > + } else if (status != QCRYPTO_TLS_BYE_COMPLETE) { > > > + /* BYE event must be synchronous, retry immediately */ > > > + trace_qio_channel_tls_bye_retry(ioc, status); > > > + goto retry; > > > } > > > > We cannot do this. If the gnutls_bye() API needs to perform > > socket I/O, and so when we're running over a non-blocking > > socket we must expect EAGAIN. With this handling, QEMU will > > busy loop burning 100% CPU when the socket is not ready. > > Right. That was the plan when drafting this, the hope is spinning will > almost not happen, and even if it happens it'll finish soon (migration is > completing, it means network mustn't be so bad), unless the network is > stuck exactly at when we send the bye(). Don't forget that the machine can be running 5 migrations of different VMs concurrently, and so may not be as quick to finish sending traffic as we expect. Since QEMU's mig protocol is essentially undirectional, I wonder if the send buffer could still be full of VMstate data waiting to be sent ? Perhaps its fine, but I don't like relying on luck, or hard-to-prove scenarios. > > A second point is that from a QIOChannel POV, we need to > > ensure that all APIs can be used in a non-blocking scenario. > > This is why in the QIOChannelSocket impl connect/listen APIs > > we provide both _sync and _async variants of the APIs, or > > in the QIOChannelTLS impl, the handshake API is always > > async with a callback to be invokved on completion. > > I agree. The issue is if so, migration code needs to be always be prepared > with a possible async op even if in 99.9999% cases it won't happen... we > need to complicate the multifd logic a lot for this, but the gain is > little.. > > This series still used patch 1 to fix the problem (rather than do real BYE > on preempt channels, for example) only because it's the easiest, after all > it's still a contract in tls channel impl to allow premature termination > for explicit shutdown()s on the host. > > If we want to do 100% graceful shutdowns, we'll need to apply this to all > channels, and the async-possible model can definitely add more complexity > more than multifd. I hope it won't be necessary.. but just to mention it. Even if the migration code is relying on non-blocking sockets for most of its work, at the time we're ready to invoke "bye", perhaps the migration code could simply call qio_channel_set_blocking(ioc, true) to switch the socket over to blocking mode. > > The QIOChanel 'bye' method is flawed in that it is > > asynchronous, but has no callback for completion. > > > > If migration is /always/ using a blocking socket for the > > TLS channels this isn't a problem as gnutls will complete > > immediately, but if any async sockets are used we have no > > way to wait for completion. This requires improving the > > API design in some manner. > > I recall one of your future series on TLS would start to enable async for > all channels? In all cases, we definitely don't want to have this call to > be relevant to the blocking mode of the channels. > > Would it make sense to introduce a _sync() version of it, but keep the > original bye(), leaving the rest until a real async user appears? > > I can also at least drop this patch as of now, because we can still wish it > almost always be synchronous. However we have risk forgetting that forever > and hit it a few years later.. If we leave the current code as-is, and relying on migration switching to blocking mode first before calling bye, we'll be ok With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|