From 7730f32c281f6005b5d35e0ba4b537a47d2dbf32 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Fri, 14 Oct 2022 15:47:15 +0200
Subject: 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>
---
 util/qemu-thread-posix.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

(limited to 'util/qemu-thread-posix.c')

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index ac1d56e673..bae938c670 100644
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -16,6 +16,7 @@
 #include "qemu/notify.h"
 #include "qemu-thread-common.h"
 #include "qemu/tsan.h"
+#include "qemu/bitmap.h"
 
 static bool name_threads;
 
@@ -552,6 +553,75 @@ void qemu_thread_create(QemuThread *thread, const char *name,
     pthread_attr_destroy(&attr);
 }
 
+int qemu_thread_set_affinity(QemuThread *thread, unsigned long *host_cpus,
+                             unsigned long nbits)
+{
+#if defined(CONFIG_PTHREAD_AFFINITY_NP)
+    const size_t setsize = CPU_ALLOC_SIZE(nbits);
+    unsigned long value;
+    cpu_set_t *cpuset;
+    int err;
+
+    cpuset = CPU_ALLOC(nbits);
+    g_assert(cpuset);
+
+    CPU_ZERO_S(setsize, cpuset);
+    value = find_first_bit(host_cpus, nbits);
+    while (value < nbits) {
+        CPU_SET_S(value, setsize, cpuset);
+        value = find_next_bit(host_cpus, nbits, value + 1);
+    }
+
+    err = pthread_setaffinity_np(thread->thread, setsize, cpuset);
+    CPU_FREE(cpuset);
+    return err;
+#else
+    return -ENOSYS;
+#endif
+}
+
+int qemu_thread_get_affinity(QemuThread *thread, unsigned long **host_cpus,
+                             unsigned long *nbits)
+{
+#if defined(CONFIG_PTHREAD_AFFINITY_NP)
+    unsigned long tmpbits;
+    cpu_set_t *cpuset;
+    size_t setsize;
+    int i, err;
+
+    tmpbits = CPU_SETSIZE;
+    while (true) {
+        setsize = CPU_ALLOC_SIZE(tmpbits);
+        cpuset = CPU_ALLOC(tmpbits);
+        g_assert(cpuset);
+
+        err = pthread_getaffinity_np(thread->thread, setsize, cpuset);
+        if (err) {
+            CPU_FREE(cpuset);
+            if (err != -EINVAL) {
+                return err;
+            }
+            tmpbits *= 2;
+        } else {
+            break;
+        }
+    }
+
+    /* Convert the result into a proper bitmap. */
+    *nbits = tmpbits;
+    *host_cpus = bitmap_new(tmpbits);
+    for (i = 0; i < tmpbits; i++) {
+        if (CPU_ISSET(i, cpuset)) {
+            set_bit(i, *host_cpus);
+        }
+    }
+    CPU_FREE(cpuset);
+    return 0;
+#else
+    return -ENOSYS;
+#endif
+}
+
 void qemu_thread_get_self(QemuThread *thread)
 {
     thread->thread = pthread_self();
-- 
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