[Qemu-devel] [PATCH v3 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD

[Jan Kiszka]

Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between
checking for exit_request on vcpu entry and timer signals arriving
before KVM starts to catch them. Plug it by blocking both timer related
signals also on !CONFIG_IOTHREAD and process those via signalfd.

As this fix depends on real signalfd support (otherwise the timer
signals only kick the compat helper thread, and the main thread hangs),
we need to detect the invalid constellation and abort configure.

Signed-off-by: Jan Kiszka <address@hidden>
CC: Stefan Hajnoczi <address@hidden>
---

I don't want to invest that much into !IOTHREAD anymore, so let's see if
the proposed catch&abort is acceptable.

 configure |    6 ++++++
 cpus.c    |   20 ++++++++++++++++++++
 2 files changed, 26 insertions(+), 0 deletions(-)

diff --git a/configure b/configure
index 4673bf0..368ca8a 100755
--- a/configure
+++ b/configure
@@ -2056,6 +2056,12 @@ EOF
 
 if compile_prog "" "" ; then
   signalfd=yes
+elif test "$kvm" = "yes" -a "$io_thread" != "yes"; then
+  echo
+  echo "ERROR: Host kernel lacks signalfd() support,"
+  echo "but KVM depends on it when the IO thread is disabled."
+  echo
+  exit 1
 fi
 
 # check if eventfd is supported
diff --git a/cpus.c b/cpus.c
index fc3f222..f9d9f9e 100644
--- a/cpus.c
+++ b/cpus.c
@@ -254,6 +254,10 @@ static void qemu_kvm_init_cpu_signals(CPUState *env)
     pthread_sigmask(SIG_BLOCK, NULL, &set);
     sigdelset(&set, SIG_IPI);
     sigdelset(&set, SIGBUS);
+#ifndef CONFIG_IOTHREAD
+    sigdelset(&set, SIGIO);
+    sigdelset(&set, SIGALRM);
+#endif
     r = kvm_set_signal_mask(env, &set);
     if (r) {
         fprintf(stderr, "kvm_set_signal_mask: %s\n", strerror(-r));
@@ -351,6 +355,12 @@ static void qemu_kvm_eat_signals(CPUState *env)
             exit(1);
         }
     } while (sigismember(&chkset, SIG_IPI) || sigismember(&chkset, SIGBUS));
+
+#ifndef CONFIG_IOTHREAD
+    if (sigismember(&chkset, SIGIO) || sigismember(&chkset, SIGALRM)) {
+        qemu_notify_event();
+    }
+#endif
 }
 
 #else /* _WIN32 */
@@ -398,6 +408,14 @@ int qemu_init_main_loop(void)
     int ret;
 
     sigemptyset(&blocked_signals);
+    if (kvm_enabled()) {
+        /*
+         * We need to process timer signals synchronously to avoid a race
+         * between exit_request check and KVM vcpu entry.
+         */
+        sigaddset(&blocked_signals, SIGIO);
+        sigaddset(&blocked_signals, SIGALRM);
+    }
 
     ret = qemu_signalfd_init(blocked_signals);
     if (ret) {
@@ -535,6 +553,8 @@ static sigset_t block_io_signals(void)
     sigaddset(&set, SIGALRM);
     sigaddset(&set, SIG_IPI);
     sigaddset(&set, SIGBUS);
+    sigaddset(&set, SIGIO);
+    sigaddset(&set, SIGALRM);
     pthread_sigmask(SIG_BLOCK, &set, NULL);
 
     memset(&action, 0, sizeof(action));
-- 
1.7.1