[pve-devel] [PATCH v2 qemu-server] fix #4501: TCP migration: start vm: move port reservation and usage closer together

[Fiona Ebner]

Currently, volume activation, PCI reservation and resetting systemd
scope happen in between, so the 5 second expiretime used for port
reservation is not always enough.

It's possible to defer telling QEMU where it should listen for
migration and do so after it has been started via QMP. Therefore, the
port reservation can be moved very close to the actual usage.

Mentioned here for completeness and can still be done as an additional
change later if desired: next_migrate_port could be modified to
optionally return the open socket and it should be possible to pass
the file descriptor directly to QEMU, but that would require accepting
the connection before on the Perl side (otherwise leads to ENOTCONN
107). While it would avoid any races, it's not the most elegant
and the change at hand should be enough in all practical situations.

Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
---

Discussion for v1:
https://lists.proxmox.com/pipermail/pve-devel/2023-November/060149.html

Changes in v2:
    * move reservation+usage much closer together than was done in v1
      of the qemu-server patch
    * drop other partial fix attempts for pve-common

 PVE/QemuServer.pm | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
index 2063e663..3b1540b6 100644
--- a/PVE/QemuServer.pm
+++ b/PVE/QemuServer.pm
@@ -5804,10 +5804,9 @@ sub vm_start_nolock {
 		$migrate->{addr} = "[$migrate->{addr}]" if Net::IP::ip_is_ipv6($migrate->{addr});
 	    }
 
-	    my $pfamily = PVE::Tools::get_host_address_family($nodename);
-	    $migrate->{port} = PVE::Tools::next_migrate_port($pfamily);
-	    $migrate->{uri} = "tcp:$migrate->{addr}:$migrate->{port}";
-	    push @$cmd, '-incoming', $migrate->{uri};
+	    # see #4501: port reservation should be done close to usage - tell QEMU where to listen
+	    # via QMP later
+	    push @$cmd, '-incoming', 'defer';
 	    push @$cmd, '-S';
 
 	} elsif ($statefile eq 'unix') {
@@ -5991,8 +5990,15 @@ sub vm_start_nolock {
     eval { PVE::QemuServer::PCI::reserve_pci_usage($pci_reserve_list, $vmid, undef, $pid) };
     warn $@ if $@;
 
-    if (defined($res->{migrate})) {
-	print "migration listens on $res->{migrate}->{uri}\n";
+    if (defined(my $migrate = $res->{migrate})) {
+	if ($migrate->{proto} eq 'tcp') {
+	    my $nodename = nodename();
+	    my $pfamily = PVE::Tools::get_host_address_family($nodename);
+	    $migrate->{port} = PVE::Tools::next_migrate_port($pfamily);
+	    $migrate->{uri} = "tcp:$migrate->{addr}:$migrate->{port}";
+	    mon_cmd($vmid, "migrate-incoming", uri => $migrate->{uri});
+	}
+	print "migration listens on $migrate->{uri}\n";
     } elsif ($statefile) {
 	eval { mon_cmd($vmid, "cont"); };
 	warn $@ if $@;
-- 
2.39.2