accel/tcg: better handle memory constrained systems

It turns out there are some 64 bit systems that have relatively low
amounts of physical memory available to them (typically CI system).
Even with swapping available a 1GB translation buffer that fills up
can put the machine under increased memory pressure. Detect these low
memory situations and reduce tb_size appropriately.

Fixes: 600e17b ("accel/tcg: increase default code gen buffer size for 64 bit")
Signed-off-by: Alex Bennée <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Robert Foley <[email protected]>
Cc: BALATON Zoltan <[email protected]>
Cc: Christian Ehrhardt <[email protected]>
Message-Id: <[email protected]>

Author: stsquad

accel/tcg/translate-all.c

@@ -976,7 +976,12 @@ static inline size_t size_code_gen_buffer(size_t tb_size)
 {
     /* Size the buffer.  */
     if (tb_size == 0) {
-        tb_size = DEFAULT_CODE_GEN_BUFFER_SIZE;
+        size_t phys_mem = qemu_get_host_physmem();
+        if (phys_mem == 0) {
+            tb_size = DEFAULT_CODE_GEN_BUFFER_SIZE;
+        } else {
+            tb_size = MIN(DEFAULT_CODE_GEN_BUFFER_SIZE, phys_mem / 8);
+        }
     }
     if (tb_size < MIN_CODE_GEN_BUFFER_SIZE) {
         tb_size = MIN_CODE_GEN_BUFFER_SIZE;