Re: [PATCH v5 1/2] migration: Make dirty_pages_rate atomic

[Paolo Bonzini]

On 4/26/23 22:10, Juan Quintela wrote:
> In this case we use qatomic operations instead of Stat64 wrapper
> because there is no stat64_set().  Defining the 64 bit wrapper is
> trivial. The one without atomics is more interesting.

This does not work if CONFIG_ATOMIC64 is not defined.

I actually have stat64_set in a patchset that I still have to submit, so 
you can reuse it:

diff --git a/include/qemu/stats64.h b/include/qemu/stats64.h
index 802402254b6f..99b5cb724a89 100644
--- a/include/qemu/stats64.h
+++ b/include/qemu/stats64.h
@@ -40,6 +40,11 @@ static inline uint64_t stat64_get(const Stat64 *s)
     return qatomic_read__nocheck(&s->value);
 }

+static inline void stat64_set(Stat64 *s, uint64_t value)
+{
+    qatomic_set__nocheck(&s->value, value);
+}
+
 static inline void stat64_add(Stat64 *s, uint64_t value)
 {
     qatomic_add(&s->value, value);
@@ -62,6 +67,7 @@ static inline void stat64_max(Stat64 *s,
 }
 #else
 uint64_t stat64_get(const Stat64 *s);
+void stat64_set(Stat64 *s, uint64_t value);
 bool stat64_min_slow(Stat64 *s, uint64_t value);
 bool stat64_max_slow(Stat64 *s, uint64_t value);
 bool stat64_add32_carry(Stat64 *s, uint32_t low, uint32_t high);
diff --git a/util/stats64.c b/util/stats64.c
index 897613c94965..efa04d5759f0 100644
--- a/util/stats64.c
+++ b/util/stats64.c
@@ -57,6 +57,17 @@ uint64_t stat64_get(const Stat64 *s)
     return ((uint64_t)high << 32) | low;
 }

+void stat64_set(Stat64 *s, uint64_t val)
+{
+    while (!stat64_wrtrylock(s)) {
+        cpu_relax();
+    }
+
+    qatomic_set(&s->high, val >> 32);
+    qatomic_set(&s->low, val);
+    stat64_wrunlock(s);
+}
+
 bool stat64_add32_carry(Stat64 *s, uint32_t low, uint32_t high)
 {
     uint32_t old;


Paolo