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 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
