[PATCH v1 2/7] accel/tcg: suppress IRQ check for special TBs

["Alex Bennée" <alex.bennee@linaro.org>]

Generally when we set cpu->cflags_next_tb it is because we want to
carefully control the execution of the next TB. Currently there is a
race that causes cflags_next_tb to get ignored if an IRQ is processed
before we execute any actual instructions.

To avoid this we introduce a new compiler flag: CF_NOIRQ to suppress
this check in the generated code so we know we will definitely execute
the next block.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Pavel Dovgalyuk <pavel.dovgalyuk@ispras.ru>
Fixes: https://gitlab.com/qemu-project/qemu/-/issues/245
---
 include/exec/exec-all.h   |  1 +
 include/exec/gen-icount.h | 21 +++++++++++++++++----
 accel/tcg/cpu-exec.c      | 14 ++++++++++++++
 3 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index 6bb2a0f7ec..35d8e93976 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -503,6 +503,7 @@ struct TranslationBlock {
 #define CF_USE_ICOUNT    0x00020000
 #define CF_INVALID       0x00040000 /* TB is stale. Set with @jmp_lock held */
 #define CF_PARALLEL      0x00080000 /* Generate code for a parallel context */
+#define CF_NOIRQ         0x00100000 /* Generate an uninterruptible TB */
 #define CF_CLUSTER_MASK  0xff000000 /* Top 8 bits are cluster ID */
 #define CF_CLUSTER_SHIFT 24
 
diff --git a/include/exec/gen-icount.h b/include/exec/gen-icount.h
index 610cba58fe..c57204ddad 100644
--- a/include/exec/gen-icount.h
+++ b/include/exec/gen-icount.h
@@ -21,7 +21,6 @@ static inline void gen_tb_start(const TranslationBlock *tb)
 {
     TCGv_i32 count;
 
-    tcg_ctx->exitreq_label = gen_new_label();
     if (tb_cflags(tb) & CF_USE_ICOUNT) {
         count = tcg_temp_local_new_i32();
     } else {
@@ -42,7 +41,19 @@ static inline void gen_tb_start(const TranslationBlock *tb)
         icount_start_insn = tcg_last_op();
     }
 
-    tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, tcg_ctx->exitreq_label);
+    /*
+     * Emit the check against icount_decr.u32 to see if we should exit
+     * unless we suppress the check with CF_NOIRQ. If we are using
+     * icount and have suppressed interruption the higher level code
+     * should have ensured we don't run more instructions than the
+     * budget.
+     */
+    if (tb_cflags(tb) & CF_NOIRQ) {
+        tcg_ctx->exitreq_label = NULL;
+    } else {
+        tcg_ctx->exitreq_label = gen_new_label();
+        tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, tcg_ctx->exitreq_label);
+    }
 
     if (tb_cflags(tb) & CF_USE_ICOUNT) {
         tcg_gen_st16_i32(count, cpu_env,
@@ -74,8 +85,10 @@ static inline void gen_tb_end(const TranslationBlock *tb, int num_insns)
                            tcgv_i32_arg(tcg_constant_i32(num_insns)));
     }
 
-    gen_set_label(tcg_ctx->exitreq_label);
-    tcg_gen_exit_tb(tb, TB_EXIT_REQUESTED);
+    if (tcg_ctx->exitreq_label) {
+        gen_set_label(tcg_ctx->exitreq_label);
+        tcg_gen_exit_tb(tb, TB_EXIT_REQUESTED);
+    }
 }
 
 #endif
diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 9cb892e326..9e3ed42ceb 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -721,6 +721,15 @@ static inline bool need_replay_interrupt(int interrupt_request)
 static inline bool cpu_handle_interrupt(CPUState *cpu,
                                         TranslationBlock **last_tb)
 {
+    /*
+     * If we have special cflags lets not get distracted with IRQs. We
+     * shall exit the loop as soon as the next TB completes what it
+     * needs to do.
+     */
+    if (cpu->cflags_next_tb != -1) {
+        return false;
+    }
+
     /* Clear the interrupt flag now since we're processing
      * cpu->interrupt_request and cpu->exit_request.
      * Ensure zeroing happens before reading cpu->exit_request or
@@ -954,11 +963,16 @@ int cpu_exec(CPUState *cpu)
              * after-access watchpoints.  Since this request should never
              * have CF_INVALID set, -1 is a convenient invalid value that
              * does not require tcg headers for cpu_common_reset.
+             *
+             * As we don't want this special TB being interrupted by
+             * some sort of asynchronous event we apply CF_NOIRQ to
+             * disable the usual event checking.
              */
             cflags = cpu->cflags_next_tb;
             if (cflags == -1) {
                 cflags = curr_cflags(cpu);
             } else {
+                cflags |= CF_NOIRQ;
                 cpu->cflags_next_tb = -1;
             }
 
-- 
2.30.2