util: Introduce qemu_thread_set_affinity() and qemu_thread_get_affinity()

Usually, we let upper layers handle CPU pinning, because
pthread_setaffinity_np() (-> sched_setaffinity()) is blocked via
seccomp when starting QEMU with
    -sandbox enable=on,resourcecontrol=deny

However, we want to configure and observe the CPU affinity of threads
from QEMU directly in some cases when the sandbox option is either not
enabled or not active yet.

So let's add a way to configure CPU pinning via
qemu_thread_set_affinity() and obtain CPU affinity via
qemu_thread_get_affinity() and implement them under POSIX using
pthread_setaffinity_np() + pthread_getaffinity_np().

Implementation under Windows is possible using SetProcessAffinityMask()
+ GetProcessAffinityMask(), however, that is left as future work.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <20221014134720.168738-3-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/meson.build b/meson.build
index b686dfef75..3e0aa4925d 100644
--- a/meson.build
+++ b/meson.build
@@ -2114,7 +2114,23 @@ config_host_data.set('CONFIG_PTHREAD_CONDATTR_SETCLOCK', cc.links(gnu_source_pre
     pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
     return 0;
   }''', dependencies: threads))
+config_host_data.set('CONFIG_PTHREAD_AFFINITY_NP', cc.links(gnu_source_prefix + '''
+  #include <pthread.h>
 
+  static void *f(void *p) { return NULL; }
+  int main(void)
+  {
+    int setsize = CPU_ALLOC_SIZE(64);
+    pthread_t thread;
+    cpu_set_t *cpuset;
+    pthread_create(&thread, 0, f, 0);
+    cpuset = CPU_ALLOC(64);
+    CPU_ZERO_S(setsize, cpuset);
+    pthread_setaffinity_np(thread, setsize, cpuset);
+    pthread_getaffinity_np(thread, setsize, cpuset);
+    CPU_FREE(cpuset);
+    return 0;
+  }''', dependencies: threads))
 config_host_data.set('CONFIG_SIGNALFD', cc.links(gnu_source_prefix + '''
   #include <sys/signalfd.h>
   #include <stddef.h>