Re: [Qemu-devel] [PATCH v2 0/3] Fix exceptions handling for MIPS and i386

[Aurelien Jarno <aurelien@aurel32.net>]

On 2015-06-18 10:16, Aurelien Jarno wrote:
> On x86, this patch brings a 5% boot time improvement on MIPS. One of the
> reason is that the TCG code generator has a good knowledge about which
> TCG ops or helpers can trigger an exception, so it can optimize out part
> of the instructions saving the CPU state. I guess that the host CPUs have
> also evolved over the time, now being superscalar and out-of-order so
> that saving the CPU state can be done "in background". Also it's just a
> quick and dirty patch, we can probably even do better.
> 
> All of that to say that I am worried for the performances to see more
> paths through the retranslation code, especially on MIPS as it seems to
> be costly. That said I haven't really look in details at other targets,
> nor hosts.

For an i386 guest still on an x86 host, I get a 4% slower boot time by
not using retranslation (see patch below). This is not that much
compared to the complexity retranslation bring us.

diff --git a/target-i386/translate.c b/target-i386/translate.c
index 58b1959..de65bba 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -8001,6 +8001,9 @@ static inline void gen_intermediate_code_internal(X86CPU *cpu,
 
     gen_tb_start(tb);
     for(;;) {
+        gen_update_cc_op(dc);
+        gen_jmp_im(pc_ptr - dc->cs_base);
+
         if (unlikely(!QTAILQ_EMPTY(&cs->breakpoints))) {
             QTAILQ_FOREACH(bp, &cs->breakpoints, entry) {
                 if (bp->pc == pc_ptr &&
diff --git a/translate-all.c b/translate-all.c
index b6b0e1c..3d4c017 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -212,6 +212,8 @@ static int cpu_restore_state_from_tb(CPUState *cpu, TranslationBlock *tb,
     int64_t ti;
 #endif
 
+    return -1;
+
 #ifdef CONFIG_PROFILER
     ti = profile_getclock();
 #endif

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net