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[Qemu-devel] [PATCH v4 5/5] tcg: Rework tb_invalidated_flag

From: Sergey Fedorov
Subject: [Qemu-devel] [PATCH v4 5/5] tcg: Rework tb_invalidated_flag
Date: Thu, 21 Apr 2016 17:57:22 +0300

From: Sergey Fedorov <address@hidden>

'tb_invalidated_flag' was meant to catch two events:
 * some TB has been invalidated by tb_phys_invalidate();
 * the whole translation buffer has been flushed by tb_flush().

Then it was checked:
 * in cpu_exec() to ensure that the last executed TB can be safely
   linked to directly call the next one;
 * in cpu_exec_nocache() to decide if the original TB should be provided
   for further possible invalidation along with the temporarily
   generated TB.

It is always safe to patch an invalidated TB since it is not going to be
used anyway. It is also safe to call tb_phys_invalidate() for an already
invalidated TB. Thus, setting this flag in tb_phys_invalidate() is
simply unnecessary. Moreover, it can prevent from pretty proper linking
of TBs, if any arbitrary TB has been invalidated. So just don't touch it
in tb_phys_invalidate().

If this flag is only used to catch whether tb_flush() has been called
then rename it to 'tb_flushed'. Declare it as 'bool' and stick to using
only 'true' and 'false' to set its value. Also, instead of setting it in
tb_gen_code(), just after tb_flush() has been called, do it right inside
of tb_flush().

In cpu_exec(), this flag is used to track if tb_flush() has been called
and have made 'next_tb' (a reference to the last executed TB) invalid
for linking it to directly call the next TB. tb_flush() can be called
during the CPU execution loop from tb_gen_code(), during TB execution or
by another thread while 'tb_lock' is released. Catch for translation
buffer flush reliably by resetting this flag once before first TB lookup
and each time we find it set before trying to add a direct jump. Don't
touch in in tb_find_physical().

Each vCPU has its own execution loop in multithreaded mode and thus
should have its own copy of the flag to be able to reset it with its own
'next_tb' and don't affect any other vCPU execution thread. So make this
flag per-vCPU and move it to CPUState.

In cpu_exec_nocache(), we only need to check if tb_flush() has been
called from tb_gen_code() called by cpu_exec_nocache() itself. To do
this reliably, preserve the old value of the flag, reset it before
calling tb_gen_code(), check afterwards, and combine the saved value
back to the flag.

This patch is based on the patch "tcg: move tb_invalidated_flag to
CPUState" from Paolo Bonzini <address@hidden>.

Signed-off-by: Sergey Fedorov <address@hidden>
Signed-off-by: Sergey Fedorov <address@hidden>

Changes in v4:
 * Rebased on top of the previous patch

 cpu-exec.c              | 21 +++++++++++----------
 include/exec/exec-all.h |  2 --
 include/qom/cpu.h       |  2 ++
 translate-all.c         |  5 +----
 4 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/cpu-exec.c b/cpu-exec.c
index 08b5c21c29bb..1d12e8bc2739 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -202,16 +202,20 @@ static void cpu_exec_nocache(CPUState *cpu, int 
                              TranslationBlock *orig_tb, bool ignore_icount)
     TranslationBlock *tb;
+    bool old_tb_flushed;
     /* Should never happen.
        We only end up here when an existing TB is too long.  */
     if (max_cycles > CF_COUNT_MASK)
         max_cycles = CF_COUNT_MASK;
+    old_tb_flushed = cpu->tb_flushed;
+    cpu->tb_flushed = false;
     tb = tb_gen_code(cpu, orig_tb->pc, orig_tb->cs_base, orig_tb->flags,
                      max_cycles | CF_NOCACHE
                          | (ignore_icount ? CF_IGNORE_ICOUNT : 0));
-    tb->orig_tb = tcg_ctx.tb_ctx.tb_invalidated_flag ? NULL : orig_tb;
+    tb->orig_tb = cpu->tb_flushed ? NULL : orig_tb;
+    cpu->tb_flushed |= old_tb_flushed;
     cpu->current_tb = tb;
     /* execute the generated code */
     trace_exec_tb_nocache(tb, tb->pc);
@@ -232,8 +236,6 @@ static TranslationBlock *tb_find_physical(CPUState *cpu,
     unsigned int h;
     tb_page_addr_t phys_pc, phys_page1;
-    tcg_ctx.tb_ctx.tb_invalidated_flag = 0;
     /* find translated block using physical mappings */
     phys_pc = get_page_addr_code(env, pc);
     phys_page1 = phys_pc & TARGET_PAGE_MASK;
@@ -446,6 +448,7 @@ int cpu_exec(CPUState *cpu)
             last_tb = NULL; /* forget the last executed TB after exception */
+            cpu->tb_flushed = false; /* reset before first TB lookup */
             for(;;) {
                 interrupt_request = cpu->interrupt_request;
                 if (unlikely(interrupt_request)) {
@@ -510,14 +513,12 @@ int cpu_exec(CPUState *cpu)
                 tb = tb_find_fast(cpu);
-                /* Note: we do it here to avoid a gcc bug on Mac OS X when
-                   doing it in tb_find_slow */
-                if (tcg_ctx.tb_ctx.tb_invalidated_flag) {
-                    /* as some TB could have been invalidated because
-                       of memory exceptions while generating the code, we
-                       must recompute the hash index here */
+                if (cpu->tb_flushed) {
+                    /* Ensure that no TB jump will be modified as the
+                     * translation buffer has been flushed.
+                     */
                     last_tb = NULL;
-                    tcg_ctx.tb_ctx.tb_invalidated_flag = 0;
+                    cpu->tb_flushed = false;
                 /* see if we can patch the calling TB. When the TB
                    spans two pages, we cannot safely do a direct
diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index 736209505a68..0ba845e12b12 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -288,8 +288,6 @@ struct TBContext {
     /* statistics */
     int tb_flush_count;
     int tb_phys_invalidate_count;
-    int tb_invalidated_flag;
 void tb_free(TranslationBlock *tb);
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index b7a10f791acc..c1ae24d1fcbb 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -238,6 +238,7 @@ struct kvm_run;
  * @crash_occurred: Indicates the OS reported a crash (panic) for this CPU
  * @tcg_exit_req: Set to force TCG to stop executing linked TBs for this
  *           CPU and return to its top level loop.
+ * @tb_flushed: Indicates the translation buffer has been flushed.
  * @singlestep_enabled: Flags for single-stepping.
  * @icount_extra: Instructions until next timer event.
  * @icount_decr: Number of cycles left, with interrupt flag in high bit.
@@ -289,6 +290,7 @@ struct CPUState {
     bool stopped;
     bool crash_occurred;
     bool exit_request;
+    bool tb_flushed;
     uint32_t interrupt_request;
     int singlestep_enabled;
     int64_t icount_extra;
diff --git a/translate-all.c b/translate-all.c
index 0d5d9449dc6b..acce9396581e 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -844,6 +844,7 @@ void tb_flush(CPUState *cpu)
     CPU_FOREACH(cpu) {
         memset(cpu->tb_jmp_cache, 0, sizeof(cpu->tb_jmp_cache));
+        cpu->tb_flushed = true;
     memset(tcg_ctx.tb_ctx.tb_phys_hash, 0, 
@@ -990,8 +991,6 @@ void tb_phys_invalidate(TranslationBlock *tb, 
tb_page_addr_t page_addr)
-    tcg_ctx.tb_ctx.tb_invalidated_flag = 1;
     /* remove the TB from the hash list */
     h = tb_jmp_cache_hash_func(tb->pc);
     CPU_FOREACH(cpu) {
@@ -1081,8 +1080,6 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
         /* cannot fail at this point */
         tb = tb_alloc(pc);
         assert(tb != NULL);
-        /* Don't forget to invalidate previous TB info.  */
-        tcg_ctx.tb_ctx.tb_invalidated_flag = 1;
     gen_code_buf = tcg_ctx.code_gen_ptr;

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