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


From: Richard Henderson
Subject: Re: [PATCH v1 2/7] accel/tcg: suppress IRQ check for special TBs
Date: Wed, 24 Nov 2021 10:23:02 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0

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.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

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~



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