From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from firstgate.proxmox.com (firstgate.proxmox.com [212.224.123.68]) by lore.proxmox.com (Postfix) with ESMTPS id 84C791FF13C for ; Thu, 25 Jun 2026 09:46:28 +0200 (CEST) Received: from firstgate.proxmox.com (localhost [127.0.0.1]) by firstgate.proxmox.com (Proxmox) with ESMTP id 5574284B5; Thu, 25 Jun 2026 09:46:11 +0200 (CEST) Message-ID: <134edb5b-85ac-433e-bbf5-63a3e106db58@proxmox.com> Date: Thu, 25 Jun 2026 09:46:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH qemu-server v2 1/2] fix #5032: qemu: sync guest time on resume and snapshot of saved state To: Fiona Ebner , pve-devel@lists.proxmox.com References: <20260622134711.108611-1-j.klocker@proxmox.com> <20260622134711.108611-2-j.klocker@proxmox.com> <88e13f59-6c5a-442f-bcee-782a958ae5a1@proxmox.com> Content-Language: en-US From: Jakob Klocker In-Reply-To: <88e13f59-6c5a-442f-bcee-782a958ae5a1@proxmox.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Bm-Milter-Handled: 55990f41-d878-4baa-be0a-ee34c49e34d2 X-Bm-Transport-Timestamp: 1782373559856 X-SPAM-LEVEL: Spam detection results: 0 AWL 0.731 Adjusted score from AWL reputation of From: address BAYES_00 -1.9 Bayes spam probability is 0 to 1% DMARC_MISSING 0.1 Missing DMARC policy KAM_DMARC_STATUS 0.01 Test Rule for DKIM or SPF Failure with Strict Alignment SPF_HELO_NONE 0.001 SPF: HELO does not publish an SPF Record SPF_PASS -0.001 SPF: sender matches SPF record URIBL_BLOCKED 0.001 ADMINISTRATOR NOTICE: The query to URIBL was blocked. See http://wiki.apache.org/spamassassin/DnsBlocklists#dnsbl-block for more information. [proxmox.com,qemuconfig.pm,qemuserver.pm,agent.pm] Message-ID-Hash: YIQFEEKEVM73CZALCDC6YLKH5XKXLS3B X-Message-ID-Hash: YIQFEEKEVM73CZALCDC6YLKH5XKXLS3B X-MailFrom: j.klocker@proxmox.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; loop; banned-address; emergency; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header X-Mailman-Version: 3.3.10 Precedence: list List-Id: Proxmox VE development discussion List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: Thanks for the thorough feedback! Comments are inline. On 6/24/26 1:31 PM, Fiona Ebner wrote: > Am 22.06.26 um 3:47 PM schrieb Jakob Klocker: >> When a VM is resumed from a saved state (hibernation or snapshot with >> RAM), the guest clock may be stale. A time skew can occur when >> creating a snapshot of a running VM. >> >> Add a new agent option `set-time-on-resume` to automatically >> synchronize the guest time with the host using the QEMU Guest Agent. >> >> Trigger time synchronization: >> - after restoring a VM from a saved state or snapshot with RAM >> - after taking a snapshot >> >> This ensures consistent guest time after rollback, restore, or >> snapshot operations when the guest OS clock does not automatically >> correct itself. >> > > It's not limited to snapshots with RAM. A snapshot operation without RAM > will also pause the guest. And I think it also makes sense to do it for > resume after a pause. > Thanks for pointing that out, I missed that. I'll make sure to sync the time for snapshots without RAM as well. I will look into time sync for resume and add it in this series. I question whether we want one agent option for all the time syncs (on snapshot, rollback, pause/start) or make it more fine-grained. I'd lean toward keeping a single option. > For rollback, the agent option from the saved snapshot configuration > will be used, right? So if I created the snapshot originally with > 'set-time-on-resume', then it will always be applied upon rollback with > no way to opt-out. And vice-versa if disabled. Should we add an option > to the 'qm rollback' command to allow overriding this? Or > unconditionally use the current value for 'set-time-on-resume' instead > upon rollback? Not sure if that would be more in line with expectations > or more surprising? > > Not quite the same for resume from hibernation, but it will also be > impossible to modify the setting if already suspended, because of the > config lock. > > And for a paused VM, it would be impossible to change because 'agent' > property changes are not (yet) hot-plugged, but that might make sense to > do actually. But not sure if that would be better done as part of a > major release. > > On the other hand, resume (from pause and suspend) could also gain > support for an option to override the 'set-time-on-resume' setting. > > Maybe the command options could default to the current guest config > setting if not explicitly specified? > > What do you think? Exactly, the config from the snapshot will be used. I've discussed this offlist with colleagues before sending the patch, since I also disliked that there is no way to disable the time sync for a snapshot once it was taken. We didn't come up with any real use cases where someone would want this behavior and therefore I didn't look into this further. I forgot that someone might want to enable it for older snapshots, so the override would definitely make sense! I do agree that it feels counterintuitive, I'd expect that if the current config has sync time disabled, the rollback wouldn't use this option. I'd opt for using the current guest config, this would feel most intuitive to me, but am open to suggestions. I'll gladly add it in a v3. > >> Link: https://bugzilla.proxmox.com/show_bug.cgi?id=5032 >> >> Signed-off-by: Jakob Klocker >> Reviewed-by: Arthur Bied-Charreton >> Tested-by: Arthur Bied-Charreton >> --- >> >> changes since v1: >> - adapt warning message in the resume-from-saved-state path >> >> src/PVE/QemuConfig.pm | 7 ++++++ >> src/PVE/QemuServer.pm | 10 ++++++++ >> src/PVE/QemuServer/Agent.pm | 47 +++++++++++++++++++++++++++++++++++++ >> 3 files changed, 64 insertions(+) >> >> diff --git a/src/PVE/QemuConfig.pm b/src/PVE/QemuConfig.pm >> index c24eb835..3f0ff663 100644 >> --- a/src/PVE/QemuConfig.pm >> +++ b/src/PVE/QemuConfig.pm >> @@ -383,6 +383,13 @@ sub __snapshot_create_vol_snapshots_hook { >> next; >> } >> } >> + if ($snap->{vmstate}) { >> + my $conf = $class->load_config($vmid); >> + if (PVE::QemuServer::Agent::should_set_time_on_resume($conf->{agent})) { >> + eval { PVE::QemuServer::Agent::guest_set_time($vmid); 1 } >> + or warn "could not sync guest time after snapshot - $@"; > > Style nit: our code base uses the following pattern: > eval { func(); }; > warn "msg - $@" if $@; I missed that, thanks.> >> + } >> + } >> } >> } >> } >> diff --git a/src/PVE/QemuServer.pm b/src/PVE/QemuServer.pm >> index 55e9f520..daf10904 100644 >> --- a/src/PVE/QemuServer.pm >> +++ b/src/PVE/QemuServer.pm >> @@ -5992,6 +5992,16 @@ sub vm_start_nolock { >> ); >> } >> >> + my $from_saved_state = $resume || ($statefile && !$migratedfrom); > > Style nit: I'd kinda prefer to avoid this one-off variable. Maybe add a > short comment near the expression instead? > I'll add a comment and get rid of the variable.>> + >> + if ( >> + $from_saved_state >> + && PVE::QemuServer::Agent::should_set_time_on_resume($conf->{agent}) >> + ) { >> + eval { PVE::QemuServer::Agent::guest_set_time($vmid); 1 } >> + or warn "could not sync guest time after resume from saved state - $@"; > > Same style nit as above. > >> + } >> + >> return $res; >> } >> >> diff --git a/src/PVE/QemuServer/Agent.pm b/src/PVE/QemuServer/Agent.pm >> index be6df443..be8bbae6 100644 >> --- a/src/PVE/QemuServer/Agent.pm >> +++ b/src/PVE/QemuServer/Agent.pm >> @@ -4,6 +4,7 @@ use v5.36; >> >> use JSON; >> use MIME::Base64 qw(decode_base64 encode_base64); >> +use Time::HiRes (); > > Stlye nit: I'd use qw() for consistency with the previous line. > >> >> use PVE::JSONSchema; >> >> @@ -18,6 +19,8 @@ our @EXPORT_OK = qw( >> get_qga_key >> parse_guest_agent >> qga_check_running >> + should_set_time_on_resume >> + guest_set_time > > I don't think these are worth exporting. You already call them with the > module prefix and I do think it's helpful to be explicit like that. > Acknowledged >> ); >> >> our $agent_fmt = { >> @@ -52,6 +55,21 @@ our $agent_fmt = { >> optional => 1, >> default => 1, >> }, >> + 'set-time-on-resume' => { > > I wonder if 'set' is the best? I'm thinking about 'update' or > 'synchronize', but maybe those make it too long? 'sync'? Not sure. What > do you think? I'd opt for sync-time-on-resume - clearer than set.> >> + description => "Update the guest clock through QGA after resuming from" >> + . " hibernation or rolling back to a snapshot with RAM.", >> + verbose_description => >> + "Whether to issue the guest-set-time QEMU guest agent command after the VM" >> + . " resumes with a restored RAM state, that is, when waking from hibernation" >> + . " or after rolling back to a snapshot that includes RAM. In these cases the" >> + . " guest's clock still reflects the time the state was saved. With this" > > Nit: s/was saved/was saved at/ > > The description does not mention that it also happens after a snapshot > create operation. > >> + . " option enabled, the clock is synchronized to the host's current time," >> + . " provided the QEMU Guest Agent option is enabled in the guest's" >> + . " configuration and the agent is running inside of the guest.", >> + type => 'boolean', >> + optional => 1, >> + default => 1, > > I don't think we should enable this by default, at least not outside of > a major release. What if something relies on the current behavior and > expects a rolled-back state to have the old time? > I'll change the default to disabled, but would like this reconsidered before a major release. A VM whose clock is always in sync feels more intuitive to me.>> + }, >> type => { >> description => "Select the agent type", >> type => 'string', >> @@ -332,4 +350,33 @@ sub guest_fs_freeze_applicable($agent_str, $vmid, $logfunc = undef) { >> return 1; >> } >> >> +=head3 should_set_time_on_resume >> + >> +Returns whether the guest's clock should be synchronized to the host's via the QEMU Guest Agent >> +when the VM is resumed from saved state. Does B check whether the agent is actually running. >> + >> +=cut >> + >> +sub should_set_time_on_resume($agent_str) { >> + my $agent = parse_guest_agent($agent_str); >> + return 0 if !$agent->{enabled}; >> + return $agent->{'set-time-on-resume'} // 1; >> +} >> + >> +=head3 guest_set_time >> + >> +Sets the guest's clock via the QEMU Guest Agent's C command. If C<$time_ns> >> +(nanoseconds since the UNIX epoch, UTC) is not given, the current host time is used. Passing >> +an explicit time is required because the agent's argument-less form reads the guest's RTC, >> +which may itself be stale after a vmstate snapshot or resume. >> + >> +=cut >> + >> +sub guest_set_time($vmid, $time_ns = undef) { > > Not sure if adding the time parameter is worth it if it's not used. We > can still add it later if a caller that needs it pops up. > Will be changed in v3.>> + $time_ns //= int(Time::HiRes::time() * 1_000_000_000); >> + my $res = PVE::QemuServer::Monitor::mon_cmd($vmid, 'guest-set-time', time => $time_ns); > > Nit: I'd prefer the coercing to int() to be done here when passing the > param to mon_cmd() Acknowledged> >> + check_agent_error($res, "unable to set guest time"); >> + return; >> +} >> + >> 1; >