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