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From 0c0baf37eaaccff09f8b75015cad0dacc997ff87 Mon Sep 17 00:00:00 2001
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From: Fabiano Rosas <farosas@suse.de>
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Date: Fri, 13 Dec 2024 10:30:40 -0300
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Subject: [PATCH] migration/multifd: Fix compat with QEMU < 9.0
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Commit f5f48a7891 ("migration/multifd: Separate SYNC request with
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normal jobs") changed the multifd source side to stop sending data
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along with the MULTIFD_FLAG_SYNC, effectively introducing the concept
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of a SYNC-only packet. Relying on that, commit d7e58f412c
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("migration/multifd: Don't send ram data during SYNC") later came
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along and skipped reading data from SYNC packets.
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In a versions timeline like this:
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8.2 f5f48a7 9.0 9.1 d7e58f41 9.2
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The issue arises that QEMUs < 9.0 still send data along with SYNC, but
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QEMUs > 9.1 don't gather that data anymore. This leads to various
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kinds of migration failures due to desync/missing data.
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Stop checking for a SYNC packet on the destination and unconditionally
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unfill the packet.
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From now on:
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old -> new:
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the source sends data + sync, destination reads normally
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new -> new:
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source sends only sync, destination reads zeros
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new -> old:
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source sends only sync, destination reads zeros
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CC: qemu-stable@nongnu.org
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Fixes: d7e58f412c ("migration/multifd: Don't send ram data during SYNC")
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Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2720
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Signed-off-by: Fabiano Rosas <farosas@suse.de>
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---
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migration/multifd.c | 15 +++++++++------
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1 file changed, 9 insertions(+), 6 deletions(-)
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diff --git a/migration/multifd.c b/migration/multifd.c
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index 498e71fd102..8d0a763a720 100644
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--- a/migration/multifd.c
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+++ b/migration/multifd.c
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@@ -252,9 +252,8 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
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p->packet_num = be64_to_cpu(packet->packet_num);
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p->packets_recved++;
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- if (!(p->flags & MULTIFD_FLAG_SYNC)) {
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- ret = multifd_ram_unfill_packet(p, errp);
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- }
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+ /* Always unfill, old QEMUs (<9.0) send data along with SYNC */
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+ ret = multifd_ram_unfill_packet(p, errp);
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trace_multifd_recv_unfill(p->id, p->packet_num, p->flags,
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p->next_packet_size);
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@@ -1151,9 +1150,13 @@ static void *multifd_recv_thread(void *opaque)
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flags = p->flags;
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/* recv methods don't know how to handle the SYNC flag */
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p->flags &= ~MULTIFD_FLAG_SYNC;
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- if (!(flags & MULTIFD_FLAG_SYNC)) {
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- has_data = p->normal_num || p->zero_num;
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- }
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+
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+ /*
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+ * Even if it's a SYNC packet, this needs to be set
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+ * because older QEMUs (<9.0) still send data along with
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+ * the SYNC packet.
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+ */
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+ has_data = p->normal_num || p->zero_num;
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qemu_mutex_unlock(&p->mutex);
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} else {
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/*
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--
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GitLab
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