[Qemu-devel] [PATCH 12/15] Handle CPU interrupts by inline checking of a flag

[Michael Roth]

From: Peter Maydell <address@hidden>

Fix some of the nasty TCG race conditions and crashes by implementing
cpu_exit() as setting a flag which is checked at the start of each TB.
This avoids crashes if a thread or signal handler calls cpu_exit()
while the execution thread is itself modifying the TB graph (which
may happen in system emulation mode as well as in linux-user mode
with a multithreaded guest binary).

This fixes the crashes seen in LP:668799; however there are another
class of crashes described in LP:1098729 which stem from the fact
that in linux-user with a multithreaded guest all threads will
use and modify the same global TCG date structures (including the
generated code buffer) without any kind of locking. This means that
multithreaded guest binaries are still in the "unsupported"
category.

Signed-off-by: Peter Maydell <address@hidden>
Reviewed-by: Richard Henderson <address@hidden>
Signed-off-by: Blue Swirl <address@hidden>

(cherry picked from commit 378df4b23753a11be650af7664ca76bc75cb9f01)

Conflicts:
        exec.c
        include/qom/cpu.h
        translate-all.c
        include/exec/gen-icount.h

Signed-off-by: Michael Tokarev <address@hidden>

Conflicts:
        cpu-exec.c

Signed-off-by: Michael Roth <address@hidden>
---
 cpu-exec.c                |   25 ++++++++++++++++++++++++-
 exec.c                    |    2 +-
 include/exec/cpu-defs.h   |    1 +
 include/exec/gen-icount.h |   11 +++++++++++
 tcg/tcg.h                 |    5 +++++
 translate-all.c           |    4 ++--
 6 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/cpu-exec.c b/cpu-exec.c
index 4ffae22..1c6af24 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -63,6 +63,12 @@ static inline tcg_target_ulong cpu_tb_exec(CPUArchState 
*env, uint8_t *tb_ptr)
         TranslationBlock *tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
         cpu_pc_from_tb(env, tb);
     }
+    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_REQUESTED) {
+        /* We were asked to stop executing TBs (probably a pending
+         * interrupt. We've now stopped, so clear the flag.
+         */
+        env->tcg_exit_req = 0;
+    }
     return next_tb;
 }
 
@@ -606,7 +612,20 @@ int cpu_exec(CPUArchState *env)
                     tc_ptr = tb->tc_ptr;
                     /* execute the generated code */
                     next_tb = cpu_tb_exec(env, tc_ptr);
-                    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_ICOUNT_EXPIRED) {
+                    switch (next_tb & TB_EXIT_MASK) {
+                    case TB_EXIT_REQUESTED:
+                        /* Something asked us to stop executing
+                         * chained TBs; just continue round the main
+                         * loop. Whatever requested the exit will also
+                         * have set something else (eg exit_request or
+                         * interrupt_request) which we will handle
+                         * next time around the loop.
+                         */
+                        tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
+                        next_tb = 0;
+                        break;
+                    case TB_EXIT_ICOUNT_EXPIRED:
+                    {
                         /* Instruction counter expired.  */
                         int insns_left;
                         tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
@@ -630,6 +649,10 @@ int cpu_exec(CPUArchState *env)
                             next_tb = 0;
                             cpu_loop_exit(env);
                         }
+                        break;
+                    }
+                    default:
+                        break;
                     }
                 }
                 env->current_tb = NULL;
diff --git a/exec.c b/exec.c
index b85508b..371713a 100644
--- a/exec.c
+++ b/exec.c
@@ -493,7 +493,7 @@ void cpu_reset_interrupt(CPUArchState *env, int mask)
 void cpu_exit(CPUArchState *env)
 {
     env->exit_request = 1;
-    cpu_unlink_tb(env);
+    env->tcg_exit_req = 1;
 }
 
 void cpu_abort(CPUArchState *env, const char *fmt, ...)
diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
index 2911b9f..07fce69 100644
--- a/include/exec/cpu-defs.h
+++ b/include/exec/cpu-defs.h
@@ -161,6 +161,7 @@ typedef struct CPUWatchpoint {
     uint32_t halted; /* Nonzero if the CPU is in suspend state */       \
     uint32_t interrupt_request;                                         \
     volatile sig_atomic_t exit_request;                                 \
+    volatile sig_atomic_t tcg_exit_req;                                 \
     CPU_COMMON_TLB                                                      \
     struct TranslationBlock *tb_jmp_cache[TB_JMP_CACHE_SIZE];           \
     /* buffer for temporaries in the code generator */                  \
diff --git a/include/exec/gen-icount.h b/include/exec/gen-icount.h
index c858a73..f45f975 100644
--- a/include/exec/gen-icount.h
+++ b/include/exec/gen-icount.h
@@ -7,10 +7,18 @@
 
 static TCGArg *icount_arg;
 static int icount_label;
+static int exitreq_label;
 
 static inline void gen_icount_start(void)
 {
     TCGv_i32 count;
+    TCGv_i32 flag;
+
+    exitreq_label = gen_new_label();
+    flag = tcg_temp_local_new_i32();
+    tcg_gen_ld_i32(flag, cpu_env, offsetof(CPUArchState, tcg_exit_req));
+    tcg_gen_brcondi_i32(TCG_COND_NE, flag, 0, exitreq_label);
+    tcg_temp_free_i32(flag);
 
     if (!use_icount)
         return;
@@ -29,6 +37,9 @@ static inline void gen_icount_start(void)
 
 static void gen_icount_end(TranslationBlock *tb, int num_insns)
 {
+    gen_set_label(exitreq_label);
+    tcg_gen_exit_tb((tcg_target_long)tb + TB_EXIT_REQUESTED);
+
     if (use_icount) {
         *icount_arg = num_insns;
         gen_set_label(icount_label);
diff --git a/tcg/tcg.h b/tcg/tcg.h
index 10eb3f4..34b2ca8 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -686,6 +686,10 @@ extern uint8_t *code_gen_prologue;
  *        would hit zero midway through it. In this case the next-TB pointer
  *        returned is the TB we were about to execute, and the caller must
  *        arrange to execute the remaining count of instructions.
+ *  3:    we stopped because the CPU's exit_request flag was set
+ *        (usually meaning that there is an interrupt that needs to be
+ *        handled). The next-TB pointer returned is the TB we were
+ *        about to execute when we noticed the pending exit request.
  *
  * If the bottom two bits indicate an exit-via-index then the CPU
  * state is correctly synchronised and ready for execution of the next
@@ -702,6 +706,7 @@ extern uint8_t *code_gen_prologue;
 #define TB_EXIT_IDX0 0
 #define TB_EXIT_IDX1 1
 #define TB_EXIT_ICOUNT_EXPIRED 2
+#define TB_EXIT_REQUESTED 3
 
 #if !defined(tcg_qemu_tb_exec)
 # define tcg_qemu_tb_exec(env, tb_ptr) \
diff --git a/translate-all.c b/translate-all.c
index bf1db09..1288b2a 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -1476,7 +1476,7 @@ static void tcg_handle_interrupt(CPUArchState *env, int 
mask)
             cpu_abort(env, "Raised interrupt while not in I/O function");
         }
     } else {
-        cpu_unlink_tb(env);
+        env->tcg_exit_req = 1;
     }
 }
 
@@ -1617,7 +1617,7 @@ void dump_exec_info(FILE *f, fprintf_function cpu_fprintf)
 void cpu_interrupt(CPUArchState *env, int mask)
 {
     env->interrupt_request |= mask;
-    cpu_unlink_tb(env);
+    env->tcg_exit_req = 1;
 }
 
 /*
-- 
1.7.9.5