On 11/23/21 9:57 PM, Alex Bennée wrote:
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.ben...@linaro.org>
Cc: Pavel Dovgalyuk <pavel.dovgal...@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
Split patch here, I think.
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.
+ */
We will *probably* exit the loop, I think.
With watchpoints, we expect to perform the same memory operation, which is expected to hit
the watchpoint_hit check in cpu_check_watchpoint, which will raise CPU_INTERRUPT_DEBUG.
With SMC, we flush all TBs from the current page, re-execute one insn, and then (probably)
have to exit to generate the next tb.
With icount, in cpu_loop_exec_tb, we have no idea what's coming; we only know that we want
no more than N insns to execute.
None of which directly exit the loop -- we need the IRQ check at the beginning of the
*next* TB to exit the loop.
If we want to force an exit from the loop, we need CF_NO_GOTO_TB | CF_NO_GOTO_PTR. Which
is probably a good idea, at least for watchpoints; exit_tb is the fastest way out of the
TB to recognize the debug interrupt.
The icount usage I find a bit odd. I don't think that we should suppress an IRQ in that
case -- we can have up to 510 insns outstanding on icount_budget, which is clearly far too
many to have IRQs disabled. I think we should not use cflags_next_tb for this at all, but
should apply the icount limit later somehow, because an IRQ *could* be recognized
immediately, with the first TB of the interrupt handler running with limited budget, and
the icount tick being recognized at that point.
+ * 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;
}
Is it clearer to add NOIRQ here, or back where we set cflags_next_tb in the
first place?
r~
