[PATCH v2 3/3] tcg: Defer tb_flush when initial thread region alloc fails

A vCPU hotplug may happen at any time.  When the new thread is
started, the region pool may be exhausted.  Do not abort.

Rename tcg_region_thread_initial_alloc to differentiate it
from tcg_region_initial_alloc__locked.  The renamed function
now uses tcg_region_alloc__locked and is prepared for failure.

In tcg_tb_alloc, allow code_gen_ptr to be NULL.  Treat that as
any other region exhaustion.  Reorg with while instead of goto.

Reported-by: Anushree Mathur <[email protected]>
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/2984
Signed-off-by: Richard Henderson <[email protected]>
---
 tcg/tcg-internal.h |  2 +-
 tcg/region.c       | 24 ++++++++++++++++++++++--
 tcg/tcg.c          | 22 ++++++++++++++--------
 3 files changed, 37 insertions(+), 11 deletions(-)

diff --git a/tcg/tcg-internal.h b/tcg/tcg-internal.h
index c0997ab224..e35440dc8c 100644
--- a/tcg/tcg-internal.h
+++ b/tcg/tcg-internal.h
@@ -42,7 +42,7 @@ extern unsigned int tcg_max_ctxs;
 
 void tcg_region_init(size_t tb_size, int splitwx, unsigned max_threads);
 bool tcg_region_alloc(TCGContext *s);
-void tcg_region_initial_alloc(TCGContext *s);
+void tcg_region_thread_initial_alloc(TCGContext *s);
 void tcg_region_prologue_set(TCGContext *s);
 
 static inline void *tcg_call_func(TCGOp *op)
diff --git a/tcg/region.c b/tcg/region.c
index 8ee8c39c43..8bd378b212 100644
--- a/tcg/region.c
+++ b/tcg/region.c
@@ -396,11 +396,31 @@ static void tcg_region_initial_alloc__locked(TCGContext *s)
     g_assert(ok);
 }
 
-void tcg_region_initial_alloc(TCGContext *s)
+void tcg_region_thread_initial_alloc(TCGContext *s)
 {
+    bool ok;
+
     qemu_mutex_lock(&region.lock);
-    tcg_region_initial_alloc__locked(s);
+    ok = tcg_region_alloc__locked(s);
     qemu_mutex_unlock(&region.lock);
+
+    /*
+     * A vCPU hotplug may happen at any time.  When the new thread is
+     * started, the region pool may be exhausted.  At this point in
+     * the new thread call stack, we are not in a position to fix this.
+     * Leave code_gen_ptr NULL, so that this thread's first call to
+     * tcg_tb_alloc() returns NULL, so that the translator performs
+     * a tb_flush() and retry.
+     *
+     * During the tb_flush(), tcg_region_reset_all() will assign a
+     * new region to all contexts, including this one.
+     */
+    if (!ok) {
+        s->code_gen_buffer = NULL;
+        s->code_gen_ptr = NULL;
+        s->code_gen_buffer_size = 0;
+        s->code_gen_highwater = NULL;
+    }
 }
 
 /* Call from a safe-work context */
diff --git a/tcg/tcg.c b/tcg/tcg.c
index af15c3d63e..db43589fa2 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -1279,7 +1279,7 @@ void tcg_register_thread(void)
     qatomic_set(&tcg_ctxs[n], s);
 
     if (n > 0) {
-        tcg_region_initial_alloc(s);
+        tcg_region_thread_initial_alloc(s);
     }
 
     tcg_ctx = s;
@@ -1830,18 +1830,24 @@ TranslationBlock *tcg_tb_alloc(TCGContext *s)
     TranslationBlock *tb;
     void *next;
 
- retry:
-    tb = (void *)ROUND_UP((uintptr_t)s->code_gen_ptr, align);
-    next = (void *)ROUND_UP((uintptr_t)(tb + 1), align);
+    while (1) {
+        tb = (void *)ROUND_UP((uintptr_t)s->code_gen_ptr, align);
 
-    if (unlikely(next > s->code_gen_highwater)) {
+        /*
+         * Note that code_gen_ptr can be NULL after vCPU hotplug.
+         * See tcg_region_thread_initial_alloc.
+         */
+        if (tb) {
+            next = (void *)ROUND_UP((uintptr_t)(tb + 1), align);
+            if (next <= s->code_gen_highwater) {
+                qatomic_set(&s->code_gen_ptr, next);
+                return tb;
+            }
+        }
         if (!tcg_region_alloc(s)) {
             return NULL;
         }
-        goto retry;
     }
-    qatomic_set(&s->code_gen_ptr, next);
-    return tb;
 }
 
 void tcg_prologue_init(void)
-- 
2.43.0