[Qemu-devel] [PATCH] stop cpus before forking.

[Glauber Costa <glommer@redhat.com>]

This patch fixes a bug that happens with kvm, irqchip-in-kernel,
while adding a netdev. Despite the situations of reproduction being
specific to kvm, I believe this fix is pretty generic, and fits here.
Specially if we ever want to have our own irqchip in kernel too.

The problem happens after the fork system call, and although it is not
100 % reproduceable, happens pretty often. After fork, the memory where
the apic is mapped is present in both processes. It ends up confusing
the vcpus somewhere in the irq <-> ack path, and qemu hangs, with no
irqs being delivered at all from that point on.

Making sure the vcpus are stopped before forking makes the problem go
away. Besides, this is a pretty unfrequent operation, which already hangs
the io-thread for a while. So it should not hurt performance.

Signed-off-by: Glauber Costa <glommer@redhat.com>
---
 net/tap.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/net/tap.c b/net/tap.c
index 0147dab..f34dd9c 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -330,6 +330,9 @@ static int launch_script(const char *setup_script, const char *ifname, int fd)
     sigaddset(&mask, SIGCHLD);
     sigprocmask(SIG_BLOCK, &mask, &oldmask);
 
+    /* make sure no cpus are running, so the apic does not
+     * get confused */
+    vm_stop(0);
     /* try to launch network script */
     pid = fork();
     if (pid == 0) {
@@ -350,6 +353,7 @@ static int launch_script(const char *setup_script, const char *ifname, int fd)
         execv(setup_script, args);
         _exit(1);
     } else if (pid > 0) {
+        vm_start();
         while (waitpid(pid, &status, 0) != pid) {
             /* loop */
         }
-- 
1.7.0.1